THE STRUCTURAL ELEMENTS

An appropriate environmental change provides a stimulus that is recognized by a receptor organ; the reaction or response that may be provoked in answer to the stimulus is performed by an effector organ (Figure 8–1). In multicellular organisms the receptor and effector organs are separate and are connected by a chain of neurons, highly specialized cells in which the general cytoplasmic properties of excitability and conductivity are developed to extreme degrees. Whatever the stimulus, the receptor neuron translates it into an electrical potential, and the message is transmitted in this coded form. The impulse travels the length of the neuron before transmission to the next cell in the chain; this may be another neuron or interneuron, but ultimately, at the end of the chain, the motor neuron will end on an effector muscle or gland cell. Neurons thus provide the basic units from which the nervous system is constructed.

The typical neuron is an elongated cell that consists of a cell body containing the nucleus (therefore known as the perikaryon) and various processes (Figure 8–2). The processes, which vary considerably in number, length, and form, are of two varieties obviously and usefully distinguished by the direction in which they transmit impulses. One variety, the dendrite, is usually multiple and transmits impulses toward the perikaryon; the other, the axon, is always single at its origin (although it may divide at some distance from the perikaryon) and conveys impulses away. The nerve cell is thus clearly polarized. The arrangement of the processes permits a simple morphological classification of neurons. Most are multipolar and possess a number (often a very large number) of branching dendrites that join the perikaryon at scattered points (see Figure 8–2). In a second type, bipolar, the dendrites join in a common trunk before reaching the perikaryon at a site remote from the origin of the axon. In the third type, unipolar, the dendrite tree and axon first combine in a single extension of the perikaryon that later branches; such neurons are also described as pseudounipolar because they initially develop as bipolar cells. Dendrites and axons are superficially alike, and both are commonly described as nerve fibers. As a general rule to which there are many exceptions, dendrites are relatively short and axons relatively long.

The different varieties of a neuron have specific distributions that are related to their particular functions. Clearly, a much-branched dendritic tree enables a neuron to receive impulses from many sources. Conversely, a much-branched axon makes connection with and stimulates many cells. The first arrangement allows a convergence of impulses from various origins; the second provides for a divergence or diffusion of a message.

Interneuronal connections are known as synapses, a term usually broadened to also include neuromuscular connections. An axon may establish synaptic connections with the bodies, dendrites, or axons of other neurons, which are varieties of synapses distinguished as axosomatic, axodendritic, and axoaxonic. Most neurons establish many synapses: some have many thousands of synaptic sites, though not in relation to so many other cells. Synapses have a variable but always complicated morphology, but only an elementary description is required here. The participating cells are neither continuous nor in direct contact but are always separated by a very narrow gap. A nerve impulse (action potential) arriving at the presynaptic part of an axon does not jump from cell to cell; instead, it prompts the release of a specific chemical transmitter substance that diffuses across the gap. When this substance arrives at the postsynaptic plasma membrane (of the following cell), it produces one of two effects: it either depolarizes the membrane, initiating a fresh impulse, which is then propagated the length of the postsynaptic cell, or it hyperpolarizes the membrane, producing a blocking or inhibitory effect. The existence of both excitatory and inhibitory synapses, sometimes on the same cell, provides a means for a great diversity of response. Many transmitter substances are known; the most common include acetylcholine, glutamate (excitatory), GABA (inhibitory) noradrenaline, serotonin, and many neuropeptides.

Neurons are supported by other specialized cells. The supporting tissue of the brain and spinal cord is known as neuroglia and comprises several cell types that we shall not distinguish. Neuroglial cells not only support the neurons but also assist in their nutrition and neurotransmission; additionally, neuroglial cells provide nerve fibers within the brain and spinal cord with cytoplasmic products that insulate them from their surroundings and prevent leakage of the impulses they convey. The insulating material, myelin, incidentally imparts a white color to nerve fibers seen en masse. Nerve fibers within peripheral trunks (outside the brain and spinal cord) receive similar insulation—of very variable thickness—from another type of supporting cell, the Schwann cell (neurolemmocytus; Figure 8–3). Peripheral nerve trunks are further protected, supported, and subdivided by connective tissue sheaths and septa, but the brain and cord, although included within a series of connective tissue investments (meninges), are not penetrated by connective tissue in this way.

Groups of perikarya are distinguished by their darker color, especially when set off by the whiteness of adjacent fiber bundles; this permits the ready distinction of the “gray” (in fact beige in the fresh specimen) and white substance of the brain and cord. Isolated neuronal aggregations within the brain are generally known as nuclei; many are too small to be distinguished by the naked eye.

Fiber bundles of common origin, destination, and function tend to be aggregated within the brain and cord into fasciculi or tracts, although the limits of these are not normally evident and can be made so only by experimental means. Most such tracts are named by the combination of their origin, employed as prefix, with their destination, employed as suffix; the significance of such names as the spinocerebellar and cerebellospinal tracts is thus revealed directly.

Neuronal aggregations on peripheral nerves may form visible swellings; they may also be distinguished by their color and texture, which are darker and firmer than those of the related nerve trunks. They are universally known as ganglia.

STIMULUS-RESPONSE APPARATUS

Having established these fundamental points, we may now return to consider the stimulus-response apparatus. In the simplest form found in mammals, this apparatus comprises five elements arranged in series: a receptor region adapted to respond to a stimulus of a particular modality* (sound, touch, and so forth); an afferent neuron that conveys an impulse centrally, toward the brain or cord; a synapse; the remainder of the efferent neuron that conveys an impulse from the center to the periphery; and an effector, which may be a muscle, gland, or neurosecretory cell (see Figure 8–1). This sequence constitutes a primary, elementary, or monosynaptic reflex arc.

The monosynaptic reflex arc is actually a most uncommon arrangement, although it does provide the basis of one familiar example, the patellar or knee-jerk reflex (Figure 8–4). This is a stretch (myotactic) reflex that, as many readers will know, can be elicited by an appropriate tap on the patellar ligament, which is the functional continuation of the quadriceps femoris muscle. The tap stretches the muscle and thus stimulates muscle spindles and other receptors within its belly and tendon; an impulse travels along afferent fibers within the femoral nerve to reach the spinal cord, where it is projected on efferent (lower motor) neurons. The axons of these neurons return within the femoral nerve, and the impulse is then projected on the constituent fibers of the muscle, stimulating their contraction to effect the abrupt extension of the joint.

In most reflexes one or more additional neurons are interposed in the chain between the afferent and efferent neurons (Figure 8–5). These are conveniently known as interneurons, although several synonyms exist. The system may still be described as simple if only an unbranched neuronal chain is involved. However, most reflexes involve more complicated circuitry in which additional neurons are stimulated (or inhibited). Collateral branching enables the exercise of a more refined control and possibly the intrusion of the activity on consciousness.

A good example of an integrated response is given by the limb of a standing animal subjected to a prick or other noxious stimulus. The limb is withdrawn by the coordinated action of the flexor muscles of several joints; these movements are facilitated by the relaxation of the previously active and antagonistic extensor muscles. The branching pathways involved in securing this response extend through several segments of the cord to reach and excite, or inhibit, the efferent neurons that supply the various muscles. At the same time, the animal has to adjust to the removal of one of its supporting props by redistributing its weight over the other limbs; the pathways necessary for this wider adjustment extend through considerable stretches of the cord, some of which cross to the contralateral side (Figure 8–6).

Coordination of the changes so that balance may be maintained involves higher centers within the brain, to which the message must ascend, in addition to integration within the cord. The process is unlikely to go unnoticed; the cortex is involved, and the animal assesses the situation and considers whether a more general response, such as flight or retaliation against the aggressor, would be appropriate. This considered response is a far cry from the simple, monosynaptic, and monosegmental response of the knee jerk and involves integrative apparatuses of various degrees of complexity, spread through the cord and brain, and drawing on those higher centers that are concerned with memory and judgment.

THE SUBDIVISIONS OF THE NERVOUS SYSTEM

Although the nervous system forms a single, integrated whole in reality, it is convenient, indeed necessary for many purposes, to divide it into parts. The most fundamental division can be made on topographical grounds, distinguishing the central nervous system (brain and spinal cord, or neuraxis) from the peripheral nervous system (the cranial, spinal, and autonomic nerve trunks with their associated ganglia). The division facilitates description but at the cost of making an artificial distinction that even assigns different parts of the same neurons to the two divisions: for example, the perikarya and axons of the efferent neurons of the patellar reflex arc.

An alternative division that would purport to have more regard to function is based on the direction in which impulses travel and on the nature of the information these impulses convey. It distinguishes afferent from efferent systems. The former conduct impulses toward the spinal cord and particular brain parts; the latter convey impulses away from these structures. Afferent pathways within the peripheral nerves are frequently termed sensory; the impulses travel from the periphery toward the brain or spinal cord. Within the cord they are more often described as ascending; the impulses travel from “lower” (more caudal) toward “higher” (more cranial) parts. Efferent pathways usually conduct impulses from “higher” to “lower” levels within the brain and cord and from these to the periphery; the alternative names to describe these systems are descending and motor. The equivalence of certain of these terms does not withstand close scrutiny, particularly when applied to integrative systems within the spinal cord; many descending fiber bundles are not motor, and many ascending bundles are not sensory.

The nature of the information that is conveyed, as well as the nature of activities that are directed, permits the further distinction of somatic and visceral nervous systems. The somatic system is concerned with those functions, like locomotion, that determine the relationship of the organism to the outside world. The visceral system is concerned with functions that relate to the internal environment: the regulation of the vascular system and heart rate, the control of glandular activity and digestive processes, and so forth. As a general but not invariable rule, there is a greater awareness and greater voluntary control of somatic than visceral functions; of course they work in close collaboration.

A more elaborate classification is possible. Afferent systems are initially divisible into somatic and visceral divisions, and these in turn are divisible into general and special subdivisions.

Somatic afferent pathways originate in receptors within the skin and deeper somatic tissues of the body wall and limbs. The pathways that arise from skin receptors are concerned with the exteroceptive sensations, such as touch, temperature, and pain, that respond to stimuli delivered from outside the organism. Receptors within the deeper tissues include the additional proprioceptive category concerned with such “deep” sensations as those that inform on the present angulation of the joints and tension within muscles and tendons and on changes in these conditions. Somatic afferent fibers are carried by all spinal nerves and by the fifth cranial (trigeminal) nerve (see Table 8–2, p. 286).

Special somatic afferent pathways have a more restricted origin within certain special sense organs: the retina of the eye and the cochlear and vestibular components of the inner ear, which are concerned with vision, hearing, and balance, respectively. The fibers concerned with vision and hearing are exteroceptive, those concerned with balance proprioceptive. Special somatic afferent fibers are thus found only within two cranial nerves, the optic and vestibulocochlear nerves.

Visceral afferent pathways originate in the (enteroceptive) receptors of vessels and glands and the viscera of the head and trunk that mostly respond to stretch and chemical stimuli. The fibers of this division are found in the cranial nerves III, V, VII, IX, and X, certain sympathetic and parasympathetic nerves, and all spinal nerves.

Special visceral afferent pathways arise from the special sense organs of smell and taste. Fibers conveying olfactory information are confined to the olfactory nerve; those conveying gustatory (taste) information are confined to a small group of cranial nerves.

Efferent systems are divided more simply.

Somatic efferent pathways lead to striated muscles of somitic and branchiomeric* origin.

Visceral efferent pathways lead to the smooth muscle of the viscera and vessels, to heart muscle, and to glands. Most of these organs receive a double innervation through the sympathetic and parasympathetic divisions of the autonomic nervous system (p. 327), which are often described as antagonistic, although “balancing” might better suggest their cooperative role. Visceral efferent fibers of the sympathetic division leave the central nervous system via the spinal nerves in the thoracolumbar regions of the cord; those of the parasympathetic division are limited to a small group of cranial nerves and to the sacral contingent of spinal nerves. However, many visceral efferent fibers later join other nerves so that they finally obtain a very widespread peripheral distribution.

SOMATOTOPY

The fibers and cell bodies within many tracts and relaying nuclei and within areas of the cerebral and cerebellar cortices on which these may project preserve very orderly point-to-point arrangements that reflect the topography of the parts of the body from which afferent impulses arise or to which efferent impulses are delivered. These do not always, or even usually, reproduce the true proportions but represent the parts of the body in relation to the densities of their innervation. The representations take the form of grotesque caricatures, sometimes known as homunculi—although animalcula would better fit veterinary anatomy—in which very sensitive parts, such as the lips and muzzle of the horse, or those capable of very refined and accurate movements, such as the fingers of a human or the prehensile tail of a monkey, are of exaggerated size. The concept of somatotopy is of great importance in the consideration of the significance of pathological lesions, in the conduct of neurosurgery, and in experimental stimulation.

INTRODUCTORY SURVEY

The brain^† and spinal cord^‡ are continuous without any clear demarcation. The brain is a very irregular organ whose shape conforms very approximately to the cranial cavity in which it is lodged, whereas the slender elongated cord has a more regular and uniform appearance.

The size of the brain bears no linear relationship to that of the animal from which it came but is relatively smaller in large species and is certainly proportionately greater in more advanced mammals. It is the relative weights that signify. The ratio of brain weight to body weight is of the order of 1 : 50, 1 : 200, and 1 : 800 in human, dog, and horse, respectively. As a rule domestication leads to reduction in weight; the process cannot be reversed by putting domesticated animals back into the wild. A more clear significance attaches to the relative development of particular parts of the brain; there is a relative preponderance of “newer” parts (in the phylogenetic sense), particularly in the cerebrum in mammals generally and in “higher” mammals specifically, when comparison is made with lower forms. The great size and complexity of the human cerebral hemispheres provide the extreme example of this evolutionary trend (Figure 8–7).

More detailed descriptions of the parts of the central nervous system are given shortly, but a first appreciation of these will be facilitated by an initial survey of the brain as a whole followed by an account of its development. Repeated references should be made to the figures so that the structures named can be located and identified.

When viewed from the dorsal direction the dominant features of the brain are the cerebral hemispheres and cerebellum; only a small part of the medulla oblongata is visible in continuity with the spinal cord (see Figure 8–20). The semiovoid cerebral hemispheres are divided from each other by a deep longitudinal fissure and from the cerebellum by a transverse fissure; when the brain is in situ, both fissures are occupied by folds of the tough dural membrane that lines the cranial cavity. Each hemisphere is molded to display ridges (gyri) and grooves (sulci) in patterns that differ significantly among the various species. The cerebellum has an even more pronounced surface marking.

The ventral aspect of the brain is flatter overall and reveals the subdivisions of the brain more clearly. The caudal part is provided by the medulla oblongata, which expands when followed forward until it terminates behind a prominent transverse ridge, the pons, which can be traced over the lateral aspect to join the cerebellum (see Figure 8–19). The midbrain in front of this, hidden in dorsal view, appears as two divergent columns, the crura cerebri (see Figure 8–19/12), which continue rostrally to disappear into the depths of the hemispheres. They are separated by the interpeduncular fossa (see Figure 8–19/13). The forebrain lies in front of this; its most prominent ventral median features are the hypothalamus (to which the hypophysis [pituitary gland] is attached by a stalk) and the crossing or chiasm formed by the optic nerves. The larger part of the forebrain is provided by the paired cerebral hemispheres, which have as their most prominent ventral features the rounded piriform lobes (see Figure 8–19/3), flanking the crura cerebri, and the olfactory tracts (see Figure 8–19/2), which originate in the olfactory bulbs that project at the rostral extremity. The superficial origins of the cranial nerves, all except the trochlear (IV) pair, are also visible on the ventral surface.

The cerebral hemispheres and cerebellum develop dorsal to the other parts, and when they are removed, all that remains is referred to as the brainstem (see Figure 8–23). This is a direct, though highly modified, continuation of the spinal cord.

DEVELOPMENT

Because the anatomy of the brain is most easily understood by reference to its development, it may be useful to give a general account of this before proceeding further; additional details will be mentioned later.

The nervous system makes a very early appearance, becoming evident at the embryonic disk stage as an elongated thickening (neural plate) of the ectoderm that overlies the notochord and paraxial mesoderm. The lateral parts of the neural plate are soon raised above the surrounding surface by growth of the underlying mesoderm and form bilateral neural folds that slope toward an axial crease, the neural groove. As the process continues, the edges of the folds become increasingly prominent and then bend inward toward each other; eventually they meet and fuse, which converts the neural groove into a neural tube (Figure 8–8). The tube, which is the primordium of the brain and spinal cord, then sinks below the surface, which is simultaneously closed above it by the fusion of the nonneural ectoderm to each side. At the same time, cells within the margins of the folds break away to form continuous cords, the neural crests, that run almost the whole length of the tube at its dorsolateral aspects. The neural crests contribute to peripheral ganglia, both somatic (dorsal root) and visceral, to the enteric nervous system, to the medullary parts of the adrenal glands, to glia, to skin melanocytes, and to a variety of craniofacial connective tissues. The sympathetic ganglia develop in the mid trunk of the embryo while neural crest cells from more cranial and more caudal regions migrate into the gut to form the enteric nervous system.

Closure of the neural tube is initially limited to the presumptive occipital region but soon spreads rostrally and caudally until only two small openings (neuropores; Figure 8–9/3,5) remain to provide communication at the surface of the embryo between the lumen of the tube and the amniotic cavity. These openings do not persist long: the rostral neuropore closes first, and the caudal one remains open for another day or two while the tube continues to lengthen at its caudal extremity by extension and subsequent infolding of the neural plate. The abnormal persistence of these openings produces relatively common defects of the brain and spinal cord in which nerve tissue may be exposed on the surface of the body. Failure at the rostral extremity leads to malformation of the forebrain and midbrain with accompanying anomalies of the skull; it is known as anencephaly and, although the term implies complete failure of brain development, it can show considerable variation in severity. Most forms are incompatible with life after birth. Failure at the caudal extremity is more common and is known as spina bifida. It is associated with defective closure of the vertebral arches. Children and young animals with this malformation may live after birth, though with severe functional disturbance; affected animals are not usually permitted to survive.

The part of the neural tube that forms the brain is wider from the outset and shows localized expansions even before the tube is completely closed. These define three primary brain vesicles: prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). The remaining, more uniform part of the tube becomes the spinal cord. The differentiation of the wall is initially similar along the length of the tube but becomes modified later in the part that becomes the brain, increasingly so toward its rostral extremity. It is convenient to consider first the differentiation of the spinal cord.

A transverse section of the tube at its formation reveals three concentric layers in its structure (Figure 8–10). These are unequally developed around the circumference, which is divisible into thick lateral parts connected by thinner roof and floor plates. The innermost layer bounding the lumen is provided by a sheet of neuroepithelial cells that persists as the ependyma lining the central canal and ventricular system of the adult cord and brain. These cells proliferate rapidly, and although some daughter cells remain as a surface lining, most migrate outward into the middle (mantle) layer of the lateral wall. These immigrant cells are neuroblasts, precursors of neurons and glia. The mantle layer itself becomes the gray substance to which the bodies of the neurons are confined. The processes from the cells within the mantle layer extend outward and form the outer (marginal) layer consisting of dendrites and axons. The marginal layer becomes the white substance of the cord in which fibers descend or ascend for various distances.

The cells of the mantle layer now become arranged in dorsal and ventral columns that bulge into the lumen of the tube, where they are separated by a longitudinal limiting groove (Figure 8–11/4). The dorsal bulge (alar plate) provides the dorsal horn or column of the gray substance of the cord; its constituent neurons are those of the afferent systems. The ventral bulge (basal plate) becomes the ventral horn or column, which is the location of the efferent neurons; both horns also contain many interneurons. Neurons with somatic functions segregate from those with visceral functions, and four groups of neurons are then arranged in dorsoventral sequence: somatic afferent, visceral afferent, visceral efferent, and somatic efferent (Figure 8–12). The roof and floor plates provide commissures through which nerve fibers pass from one side of the cord to the other.

Further growth of the alar and basal plates causes the lateral parts of the tube wall to expand outward in all directions, submerging the roof and floor plates and creating the dorsal sulcus and the ventral fissure that divide the adult cord into its right and left halves. A serial segmentation is created by the appearance of the roots of the spinal nerves. The dorsal roots are provided by neurons within the dorsal root ganglia, local condensations of neural crest cells. The axon processes of these cells extend medially to reach and penetrate the marginal layer, where they divide. Branches of these axons diffuse over several segments before entering the mantle layer to terminate on dorsal column cells; some of greater length extend to reach higher levels within the central nervous system (see Figure 8–6). The ventral roots are formed by axons of efferent neurons within the ventral column, which grow through the marginal layer to emerge on the surface of the cord, where they converge. The appearance of the roots divides the white substance into the dorsal, lateral, and ventral funiculi (Figure 8–13/7,8,9).

Although the histogenesis of the nervous system will not be described, two points must be made. In most parts of the brain the full complement of neurons is established shortly after, if not before, birth. However, contrary to former beliefs, in some regions there is a significant, more protracted postnatal recruitment in areas such as the cerebellum and hippocampus that continues into later life. Adult life is marked by a slight depletion of their number. Different authors provide very different estimates of neuronal loss in the human brain, in which the phenomenon is of the most obvious interest. The second point relates to the process of myelinization of the fibers within the central nervous system. Different tracts within the brain and cord acquire adequate insulation (essential to their function) at different stages of development. There are important species differences in this process.

The three primary brain vesicles are evident before closure of the neural tube. At this time the prosencephalon has already extended the evaginations that become the optic cups. The brain grows more rapidly than the tissues that enclose it, and the constraint that this exercises enforces a remodeling of its form. Flexures appear at three locations. The most caudal flexure is more marked in ourselves than in quadrupeds. It bends the brain ventrally at its junction with the cord. A second flexure at midbrain level is almost simultaneous and is sufficiently pronounced to bring the ventral surfaces of the forebrains and hindbrains close together; this relationship is later reversed by the third flexure, which folds the hindbrain dorsally on itself (Figure 8–14). The plan of the chief parts is completed by the appearance of paired lateral evaginations from the alar plates of the prosencephalon, directly behind the rostral limit of this part. These outgrowths, the future cerebral hemispheres, constitute the telencephalon; the unpaired median portion of the prosencephalon, hereafter known as the diencephalon, differentiates as the thalamus and related structures. The telencephalic vesicles expand in all directions but chiefly in a curve that extends dorsally and caudally to overlap the diencephalon, to which they make secondary fusions at the apposed surfaces (see Figure 8–33).

The development of the cerebellum is initially by bilateral formations in the alar plates of the metencephalon; these later extend to a median fusion.

The origin of the major components and cavities of the brain may be conveniently summarized in tabular form (Table 8–1).

Table 8–1 Derivatives of the Neural Tube

The neural tube receives an early direct envelopment provided by mesodermal cells; the forebrain region is supplemented somewhat from cells that migrate from the neural crests. They form two sheets (pia mater and arachnoid). An outer covering (dura mater) is provided by condensation from the surrounding mesoderm; it is separated from the arachnoid by a narrow space.

THE SPINAL CORD

The spinal cord (medulla spinalis) is an elongated structure that is more or less cylindrical but with some dorsoventral flattening and certain regional variations in form and dimensions. The most important of these are the thickenings (intumescentiae; Figure 8–15) of the parts that give origin to the nerves supplying the forelimbs and hindlimbs and the final caudal tapering (conus medullaris). The cord is divided into segments corresponding to the somites by the serial origins of the roots of the paired spinal nerves; the formation of these nerves has been described (p. 29). The relation of the segments to the vertebrae are considered in later chapters (see Figure 8–15).

A simple transverse section shows a central mass of gray substance perforated in the midline by a small central canal, which is the residue of the lumen of the embryonic neural tube (see Figure 8–13). The gray substance, which has a crude resemblance to a butterfly or an H, is commonly described as exhibiting dorsal and ventral horns or columns; the former is a rather misleading term as the horns extend the length of the cord (Figure 8–16). The grayness is of course produced by the restriction of the perikarya to this part. The dorsal horn corresponds to the alar plate. It contains somatic afferent neurons dorsomedially and visceral afferent neurons dorsolaterally (see Figure 8–17). The ventral horn corresponds to the basal plate; it is composed of somatic efferent neurons, which are located ventrally, and visceral efferent neurons, which form an additional lateral horn confined to the thoracolumbar and sacral regions of the cord.

The neurons within each horn are more specifically grouped according to their functional and topical associations, but this is not grossly discernible.

The white substance that envelops the gray is divided into three funiculi on each side (Figure 8–18/I,II,III). The dorsal funiculus is contained between a shallow dorsal sulcus, extended deeply by a median glial septum, and the line of origin of the dorsal roots of the spinal nerves (see Figure 8–13). The lateral funiculus is contained between the lines of the dorsal and ventral roots, and the ventral funiculus is contained between the line of the ventral roots and a ventral fissure that penetrates far into the white substance, although it leaves a considerable commissure connecting the right and left halves. This ventral fissure is occupied by a mass of pia that appears as a glistening streak on the surface of the cord.

The funiculi are composed of ascending and descending nerve fibers, of which many are grouped within bundles (fasciculi or tracts) of common origin, destination, and function (see Figure 8–18). Certain of these are mentioned later.

THE HINDBRAIN

The hindbrain (rhombencephalon) comprises the medulla oblongata, pons, and cerebellum. These parts differentiate from the caudal brain vesicle shortly after closure of the neural tube. Attenuation of the roofplate weakens the structure and causes the vesicle to flatten as the pontine flexure develops. The flattening splays the side walls outward so that the luminal surfaces come to face dorsomedially; the alar plates are now lying lateral to the basal plates (see Figure 8–24). The part caudal to the flexure (myelencephalon) becomes the medulla oblongata of adult anatomy. The rostral part develops to become metencephalon, externally marked by the pons and cerebellum. The parts of the roofplate caudal and rostral to the cerebellum remain thin and constitute the medullary vela that complete the enclosure of the lumen, now known as the fourth ventricle (see Figure 8–24).

THE MIDBRAIN

The midbrain (mesencephalon) is a short, rather constricted portion that better preserves the basic organization of the neural tube than do other parts of the brainstem.

The midbrain is exposed on the ventral surface of the intact brain, to which it contributes the crura cerebri, the interpeduncular fossa, and the superficial origin of the oculomotor nerves (III). It is concealed dorsally by the overhanging cerebral hemispheres and cerebellum. Its lumen, the aqueduct, is a simple passage joining the much larger cavities of the third and fourth ventricles. The mesencephalon has a stratified structure, comprising tectum, tegmentum, ventral tegmentum and cerebral peduncle in dorsoventral sequence (Figure 8–29). Formally, all parts except the tectum are included within the cerebral peduncles, but in practice the latter term is frequently equated with the crus cerebri, the part ventral to the tegmentum.

The tectum lies dorsal to the aqueduct. Its major features are four rounded surface swellings (see Figure 8–23). The paired caudal swellings, the caudal colliculi, are widely spaced and are joined by a substantial commissure. They are integration centers on auditory pathways (p. 300). There is a connection with the ipsilateral medial geniculate body (a swelling of the thalamus) via a distinct ridge (brachium). The rostral colliculi are placed closer together and are joined to the lateral geniculate bodies by similar but less obtrusive brachia. The rostral colliculi are staging posts on the visual pathways and are involved in somatic reflexes resulting from visual input, such as the response to being startled by a flash of intense light. They are also spatial integration centers.

The tegmentum comprises the core of the midbrain and is directly continuous with the corresponding stratum of the metencephalon. Much of it is formed by the reticular formation. The principal mesencephalic nuclei are the mesencephalic nuclei of the trigeminal nerves (V), the trochlear nuclei (IV), the principal and parasympathetic oculomotor nuclei (III), the red nuclei (named for their pronounced vascularity), and the periaqueductal gray, a core of gray substance about the aqueduct. The substantia nigra is a prominent lamina that can be identified in transverse sections by its darker color, which is due to the gradual accumulation of pigment within the constituent neurons. Like the red nucleus, it is associated with the basal nuclei (p. 291) in the control of voluntary movement.

The crura cerebri are visible on the ventral surface of the brain. They comprise fiber tracts that are in passage between the telencephalon and caudal brainstem. On emerging from the telencephalon, they converge, although they are separated by the interpeduncular fossa (Figure 8–19). The oculomotor nerves (III) emerge in this region, directly rostral to the pons.

THE FOREBRAIN

The forebrain comprises the median diencephalon and the paired cerebral hemispheres (telencephalon). The hemispheres overlap the dorsolateral aspects of the diencephalon to which they have become fused by the growth of fiber tracts across the gaps.

SOMATIC AFFERENT PATHWAYS

The designation somatic afferent is applied to those pathways of fiber tracts and intercalated nuclei that convey information from the wide array of receptors of various types that are scattered throughout the skin and the deeper somatic tissues. It excludes the special somatic afferent pathways from the eye and inner ear and, obviously, the pathways from visceral receptors.

The somatic afferent system is concerned with a variety of sensory modalities: touch, pressure, vibratory sensation, thermal sensation, pain, and the kinesthetic sensations relating to joint angulation and muscle tension. The primary neurons concerned with all these senses are located within the dorsal root ganglia of the spinal nerves (and corresponding ganglion of the trigeminal nerve where structures of the head are concerned), and their axons enter the central nervous system by way of the dorsal roots of the spinal nerves (and afferent root of the trigeminal nerve). The axons branch on entering the central nervous system. Some branches end on interneurons within the gray substance of the segment of entry or of an adjacent segment; these neurons in turn project on ventral horn cells of the same or neighboring segment, so completing the short neuron chain that provides the anatomical basis for local reflex responses. (The interneuron is omitted from the simplest reflex arc of all—that of the tendon jerk; Figure 8–4.) The ventral horn neuron whose axon ends directly on the effector is called the lower motor neuron.

Other branches of the primary axons connect directly, or through interneurons, with higher centers, thus providing pathways that initiate more complex integrated responses. The ascending pathways concerned with most sensory modalities (including a fraction of those concerned with registering joint position) ultimately reach the somatosensory area of the cerebral cortex, providing the mechanism for conscious perception. None of these ascending pathways is entirely isolated from other parts of the brain; all are variously connected to other centers by collateral branches at different levels.

SPECIAL SOMATIC AFFERENT PATHWAYS

SOMATIC MOTOR PATHWAYS

Somatic motor activity is regulated at two levels within the central nervous system by separate groups of nerve cells conveniently designated the lower and upper motor neurons.

The lower motor neurons are located within the ventral column of the gray substance of the spinal cord (Figure 8–12/4) and within the somatic motor nuclei of those cranial nerves that contain somatic efferent components. Their axons are conveyed within the spinal and relevant cranial nerves to the skeletal muscles, where each terminates on a group of muscle fibers (Figure 8–48); the size of the group varies with the precision of performance required of the particular muscle (p. 25). Lower motor neurons provide the efferent limbs of simple reflexes but are in most other circumstances directed by upper motor neurons.

The upper motor neurons are involved in more complicated reflexes and also initiate voluntary movements. They are mainly located within the motor area of the neopallium but also in other regions of the brain, including the reticular formation and red nucleus. The cortical areas allocated to neurons controlling the muscles of different parts of the body vary in extent with the importance and complexity of the movements of these parts in the habitual activities of the species; thus the hand occupies a relatively much larger area of the human cortex than that allocated to the whole limb in ungulates. Upper motor neurons do not project directly on muscle fibers but exert their control by excitation or inhibition of lower motor neurons.

The connections of the upper with lower motor neurons follow various pathways that vary considerably among species in their relative development and details of organization. The primary distinction is made between so-called pyramidal and extrapyramidal systems, although the two are coordinated and work in close collaboration. The pyramidal system is mostly concerned with the exercise of finely adjusted movements, while the extrapyramidal system is employed in the control of coarser movements, particularly in stereotyped locomotor patterns. It follows that the pyramidal system must be better developed in primates than in domestic species, which is a distinction that explains the different consequences of lesions to the pyramidal pathway. Severe damage to the pyramidal pathway produces a complete and permanent paralysis of the contralateral voluntary musculature in ourselves, while the effects in domestic species are mainly confined to disturbance of contralateral postural reactions from which partial recovery occurs after a few days. Both pyramidal and extrapyramidal systems are provided with elaborate feedback mechanisms that allow for the continuous monitoring and adjustment of motor activity.

THE HYPOTHALAMUS

An important integration center is the hypothalamus, of which the rostral part is concealed and the caudal part—exemplified by the tuber cinereum and mamillary bodies—is exposed on the surface of the brain (Figures 8–19 and 8–22). The hypothalamus includes many areas of specialized function and responsibility. A brief summation of its functions must include the control of biological rhythms, appetite, water balance, body temperature, cardiovascular performance, sexual behavior and activity, sleep, muscle tension (orexin), and emotion. Deficiencies in the orexin/hypocretin system may cause severe cataplexy and narcolepsy (also in the dog). Several hypothalamic cell groups implicated in reproductive function are sexually dimorphic. Because almost every body function has visceral implications, the hypothalamus must receive (and coordinate) information from most other parts of the nervous system, including those of ostensibly somatic function. Information on the somatic activities is projected via the basal nuclei and relays on the extrapyramidal motor pathways via the thalamic nuclei to which the somatic afferent pathways lead. Information concerning visceral function is received from mesencephalic nuclei and the reticular formation. The nucleus of the solitary tract is the principal visceral sensory nucleus that receives topographically organized input from major organ systems by way of the glossopharyngeus (IX) and vagus (X) nerves. As such it is the region of initial processing of visceral, cardiovascular and respiratory, and gustatory information. A further very important contribution comes from the telencephalon (from the prefrontal cortex) and especially from the hippocampus, via the fornix. This enables emotional inputs to be related to and coordinated with the rest. Hypothalamic input from peripheral organ systems is also possible by way of blood-borne signals.

The hypothalamus regulates activity through both nervous and humoral mechanisms, sometimes in combination. The nervous pathways extend to the brainstem and spinal cord by direct routes or by multisynaptic pathways within the reticular formation, in which final integration takes place. Other projections provide a feedback to the forebrain routed through rostral thalamic nuclei.

The humoral pathway operates through neurosecretory cells whose products may enter the bloodstream directly for general distribution or whose products may be conveyed specifically to the hypophysis by means of a system of portal vessels (see Figure 6–3).

THE HYPOPHYSIS

The hypophysis (pituitary gland; Figure 8–52) suspended below the hypothalamus by the infundibulum, consists of two parts. One, the neurohypophysis (posterior lobe), is an outgrowth of the brain itself; the other, the adenohypophysis, is developed from oral ectoderm (p. 217) and comprises anterior and intermediate lobes. Interspecific differences in the topographical interrelationship of the lobes are not of present concern (see Figure 6–2).

The three lobes produce or store several hormones (p. 217). The posterior lobe hormones (vasopressin and oxytocin) are produced by neurosecretory cells within the supraoptic and paraventricular nuclei of the hypothalamus and are conveyed along the axons for direct release into the neurohypophysial capillary bed (see Figure 6–3).

VISCERAL AFFERENT PATHWAYS

There are both “general” and special visceral afferent pathways, and the latter is concerned with taste and smell. The receptors of the general visceral afferent pathway are found within viscera and blood vessels; most are mechanoreceptors responsive to pressure, stretch, and, less commonly, flow, while a minority are chemoreceptors responsive to such stimuli as the carbon dioxide content of the blood. The fibers that convey impulses from these receptors travel within any conveniently located nerve trunk, utilizing those of mainly somatic composition as well as those whose other components are visceral efferent. The bodies of the primary neurons are located within the dorsal root ganglia of all spinal nerves (and the equivalent ganglia of certain cranial nerves); the axons project on interneurons and projection neurons within the visceral afferent column of the spinal cord and brainstem (Figure 8–12/2).

Short chains of interneurons provide for simple visceral reflexes that have their last two relays within the visceral efferent column and the peripheral autonomic ganglia. The projection neurons form ascending pathways that follow somatic systems, both lemniscal and extralemniscal, to end (like these) within nuclei of the ventrocaudal thalamus. A final projection to the cortex may give rise to conscious perception, although most visceral activity goes unnoticed. (The sense of fullness arising from digestive organs or the bladder is among the visceral activities of which awareness is most common.) Pronounced contraction and serious overdistention of visceral organs may be perceived as pain. Pain of visceral origin may be “referred” to the surface of the body, presumably as a consequence of the convergence of the cutaneous somatic and visceral afferent pathways on the same neurons at some point along their course.

The special visceral afferent pathway concerned with taste follows a similar route to that taken by the general visceral sensory modalities. The course from the taste buds within the facial, glossopharyngeal, and vagus nerves terminates in the nucleus of the solitary tract.

The more complicated olfactory pathways are described elsewhere (see Figure 8–40).

VISCERAL EFFERENT PATHWAYS

Unlike the afferent component, the efferent component of the visceral nervous system is arranged in two divisions, sympathetic and parasympathetic, distinguished by morphology, pharmacology, and physiology. The final conducting pathway of both divisions, unlike that of the somatic system, includes two motor neurons in succession: the first has its perikaryon within the central nervous system, and the second is stationed within a peripheral ganglion (Figure 8–53). The two are most frequently distinguished as preganglionic and postganglionic neurons and together are equivalent to the lower motor neuron of the somatic system.

The preganglionic neurons of the sympathetic division are located within the lateral (visceral efferent) column of the spinal cord between the first thoracic and middle lumbar segments (with some interspecific variation) (see Figure 8–74). The postganglionic neurons are found in paravertebral ganglia of the sympathetic chain or subvertebral ganglia on the aorta; both groups are relatively close to the cord.

The parasympathetic preganglionic neurons are restricted to the nuclei of origin of the oculomotor, facial, glossopharyngeal, and vagus nerves within the brainstem and the lateral columns of certain sacral segments of the cord (see Figure 8–73). The postganglionic neurons are stationed within small ganglia in close proximity to or actually incorporated within the walls of the organs they supply.

The transmitter substance at the last sympathetic relay is norepinephrine and that of the parasympathetic division is acetylcholine; both are collocated with a host of neuropeptides. The two divisions therefore react differently to autonomic stimulant and depressant drugs.

The two systems have broadly similar distributions and are frequently described as antagonist: one inhibits while the other stimulates a particular activity. This rule is less absolute than was once supposed, and their roles are better regarded as collaborative. The more diffuse anatomy of the peripheral sympathetic nerves (which are described later) and the use of norepinephrine as a transmitter indicate the more general effects produced by sympathetic activity, in contrast to those of parasympathetic activity, which are often local, effecting single specific functions.

The central control is exerted by neurons within the hypothalamus; those that influence the sympathetic division are generally caudal to those controlling the parasympathetic division. The pathways from both sets follow various routes, of which some are direct and others are via multisynaptic chains within the reticular formation.

THE LIMBIC SYSTEM

The limbic system has a complex organization and is composed of the limbic cortex and many subcortical nuclei. The cortical part forms a ring at the medial surface of the cerebral hemisphere, including, among other structures, the cingulate and supracallosal gyri, the piriform lobe, and the hippocampus. The subcortical part is composed of the hypothalamus, septal area, amygdala, habenular nuclei, and dorsal part of the mesencephalic tegmentum. There are numerous associations between these structures and other regions of the brain. The limbic system is often considered to be primarily a “visceral brain” because its major functions are expressed by visceral motor activity.

Olfactory impulses passing by way of the piriform lobes may influence many structures of the system. Of all the sensory inputs, olfaction exhibits the most profound effects on visceral motor activities that are associated with emotional behavior such as eating, rage, sexual activity, fear, and drinking. The system also receives optic, auditory, extroceptive, and introceptive stimuli.

The efferent pathways from the cortical regions involve nearly all the subcortical nuclei of the system. A major portion of the influences of the limbic cortex is mediated through the efferent systems of the amygdaloid nuclei. Electrical stimulation of the amygdala produces a wide variety of visceral and somatic reactions and many behavioral reactions such as aggression and anxiety. The types of behavior most influenced by the limbic system are those essential for the preservation of the individual or the species.

The hippocampus probably plays the predominant part in the limbic system’s control of emotional expression and behavior through regulation of autonomic, endocrine, and somatic functions. It is also concerned with memory functions, such as the processing of recently acquired memory and its more permanent consolidation. In its activities the limbic system is closely associated with the reticular formation of the brainstem.

TOPOGRAPHY

The brain and spinal cord are contained within a continuous space provided by the cranial cavity of the skull and the canal formed by successive bony rings and connecting ligaments and disks of the vertebral column.

The cranial cavity lies directly behind the nasal cavities. Smaller than is commonly supposed, its form and extent are not easily predicted from external inspection because the paranasal sinuses, horns, muscular ridges and other projections of the skull, and the temporal muscles all contribute significantly to the conformation of this part of the head. The closest agreement between the external contours and the cavity within the cranium is found in the newborn of all species; among adults this agreement is best retained in cats and in dogs of brachycephalic breeds. Fortunately, the exact location of the brain is rarely of practical significance except in the humane slaughter techniques mentioned in later chapters. It is probably sufficient to state meanwhile that the caudal limit of the cavity extends to the caudal wall of the skull—thickened by the frontal sinus in cattle—while the rostral limit shows considerable variation; it ends level with the caudal margin of the zygomatic processes of the frontal bones in dogs and cats and with the rostral level of these processes in horses and cattle, but it extends to the middle of the orbit in pigs and small ruminants.

The interior of the cranial cavity shows a fairly close correspondence with the contours of the brain, although significant intracranial space is required for the meninges and intermeningeal spaces that surround the brain and for the capacious intracranial venous sinuses. While the roof (calvaria) of the cavity remains largely undivided, the base is divided into three fossae; these need not be described in detail as the main features are depicted in Figure 8–54. The rostral fossa is formed by the sphenoid and ethmoid bones and extends to the level of the optic canals, the passages of exit of the optic nerves. It contains the olfactory bulbs, within recesses of the cribriform plate (Figure 8–54/1), and the rostral parts of the cerebral hemispheres. The middle fossa extends from the optic canals to the sharp petrosal crests (Figure 8–54/9) that project inward from the petrous temporal bones of the lateral walls. The floor is formed by the sphenoid bone, which carries the median hypophysial fossa (sella turcica) into which the hypophysis fits; it also presents various foramina of exit—the orbital fissure and the round and oval foramina—that were encountered in the previous description of the skull (p. 61). This, the widest part of the cranial cavity, contains the temporal and parietal lobes of the cerebral hemispheres. The caudal fossa extends from the caudal limit of the hypophysial fossa to the foramen magnum in the caudal wall. Its principal features are the contributions to its lateral walls made by the petrous parts of the temporal bones (each perforated by an internal acoustic meatus) and the jugular and hypoglossal foramina in the floor. The caudal fossa lodges the midbrain, pons, and medulla ventrally and the cerebellum dorsally.

The caudal, dorsal, and lateral walls of the entire cranial cavity blend together. Their most prominent internal feature is the tentorium cerebelli osseum (Figure 8–54/8), a large projection at the junction of the dorsal and caudal walls forming the middle portion of the tentorium cerebelli within the transverse fissure of the brain. It presents passages through which emerge emissary branches of the dorsal intracranial venous sinuses.

The vertebral canal is widest within the atlas and tapers rapidly within the sacrum; in between, it is most expanded where it contains the cervical and lumbar swellings of the spinal cord from which arise the nerves that form the limb plexuses (Figure 8–15). The topography of the spinal cord is of considerable importance in veterinary practice because injections into the canal are frequently made, particularly of local anesthetic solution, with the intention of blocking specific spinal nerves; in addition, there is sometimes a need to locate central nervous lesions to specific vertebral levels, which is a procedure made possible by association of specific sensory and motor deficits with particular spinal segments.

Even with the inclusion of its meningeal wrappings, the spinal cord is considerably smaller than the vertebral canal (see Figure 8–55). It is also considerably shorter. This is due to the unequal later growth of the spinal cord and vertebral column, which is an inequality that begins well before birth and continues after it. The relative shift in position (ascensus medullae) carries the segments of the cord cranially from their original positions within vertebrae of the same numerical designations. The shift of the more caudal segments is most pronounced and explains the peculiar arrangement of the associated spinal nerves. These take progressively longer courses within the canal to reach their fixed foramina of exit, forming a leash (known as the cauda equina because of a superficial resemblance to a horse’s tail) to each side of the conus medullaris (see Figure 12–9/9). The level at which the cord ends varies among species (and, in early life, with age); it is within L5 or L6 in the pig, L6 in ruminants, L6 or L7 in the dog, S2 in the horse, and rather variably between L6 and S3 in the cat (Figure 8–56).

THE MENINGES AND FLUID ENVIRONMENT

The brain and spinal cord are surrounded by three continuous membranes or meninges that exhibit certain topographical differences of importance in their cranial and vertebral parts.

The tough outermost membrane, the dura mater, is fused with the inner periosteum of the skull bones; it splits from this within the margin of the foramen magnum to form a free tube separated from the wall of the vertebral canal by a wide though varying epidural space. The epidural space is occupied by fat, more fluid in life than in the postmortem specimen, and by the internal vertebral venous plexus; the fat and vessels together cushion the spinal cord and allow it to adjust to the movements of the neck and back (see Figure 8–55). The dural tube is attached at its caudal end, where the several meninges finally combine in a fibrous strand (filum terminale) that fuses with the upper surface of the caudal vertebrae.

The fusion of the cranial dura with the periosteum obliterates the epidural space within the skull, and the cranial venous sinuses thus come to be enclosed within the thickness of the combined membrane. In addition to lining the cavity, the cranial dura forms certain folds that project inward and limit shuddering movements of the brain; these are a considerable hindrance to the removal of the intact brain at autopsy. One, the falx cerebri, extends from the dorsal and rostral cranial walls between the two cerebral hemispheres; caudally it joins a second, transverse fold, the membranous tentorium cerebelli, which separates the cerebellum from the cerebrum (Figure 8–57/7). The tentorium is ossified in its median part. A third specialization of the dura roofs the hypophysial fossa in which the hypophysis is seated, forming a diaphragm around the infundibular stalk.

A capillary space divides the dura from the subarachnoid, the first of the two more delicate inner membranes. This subdural space normally contains only a minute amount of a clear lymphlike fluid but may be enlarged by effusion of blood after an injury. The spinal part of the subdural space is crossed by a bilateral series of triangular (denticulate) ligaments that alternates with the origins of the spinal nerves; they attach the inner meninges to the dural tube and thus indirectly sling the cord (Figure 8–58/4). The outer part of the arachnoid forms a continuous membrane molded against the dural envelope. Its inner surface is joined to the pia mater by numerous trabeculae and filaments, imaginatively compared to a spider’s web (the origin of the name subarachnoid). The pia mater is directly attached to the brain and cord and follows every change in their contours. The arachnoid space, which contains the clear, watery cerebrospinal fluid, is much wider than the subdural space but less uniform, particularly in its cranial part (see Figure 8–57).

The widest parts (“cisterns”) of the cranial subarachnoid space are located between the more salient parts of the ventral surface of the brain and in the angle between the cerebellum and the dorsal aspect of the medulla. The dorsal widening, the cerebellomedullary cistern, is especially large and may be reached in the living animal if one passes a needle between the atlas and the skull (see Figure 8–57). Cisternal puncture is employed in both clinical and experimental work for obtaining samples of cerebrospinal fluid. The spinal subarachnoid space is more uniform but widens around the conus medullaris, which is a fortunate circumstance as access to the vertebral canal is easiest through the lumbosacral interarcuate space (Figure 8–59).

The pia mater is firmly attached to the outer surface of the brain and cord, and many branches from arteries within the pia penetrate the brain and cord substance. These vessels are initially enclosed by pial sleeves but these soon merge with the vascular walls. A thickening of the pia fills the ventral fissure of the spinal cord, where it appears as a glittering silver line.

All three meninges form cuffs around the roots of origin of the cranial and spinal nerves.

The cerebrospinal fluid within the arachnoid space forms a water jacket that buoys up and protects the soft brain and cord. It is largely a product of the ependymal lining of the ventricular system within the brain, and the overwhelmingly larger part is produced where this covers the choroid plexuses, vascular tufts that invaginate the ventricles (Figure 8–60/6,9). An additional contribution to the fluid is made by the pial vessels.

The ventricles are local modifications of the lumen of the neural tube; they have complicated shapes, but as these are illustrated (Figure 8–61) and as the details have little veterinary significance, they need not be described. It is more important to understand their relationship to the choroid plexuses. The plexuses of the lateral and third ventricles, which merge within the interventricular foramen, develop within a fold of the pia that becomes entrapped between the expanding telencephalic vesicles and the roof of the diencephalon (Figure 8–62). The plexuses of the fourth ventricle develop separately within the pia over the caudal medullary velum. In the course of development these plexuses thrust themselves into the lumen of the fourth ventricle; parts later reemerge into the arachnoid space by herniating through paired lateral openings in the roof (Figure 8–63).

The clear colorless cerebrospinal fluid is to a certain extent also formed from the blood plasma by ultrafiltration through the “blood–cerebrospinal fluid barrier” (blood–brain barrier) composed of vascular endothelial cells. The fluid has a higher concentration of potassium and calcium ions and a lower concentration of sodium, magnesium, and chloride ions than the plasma; it is also rather deficient in glucose and, most importantly, contains little protein because the barrier is impermeable to larger molecules, which of course include those of many antibiotic and other drugs.

In addition to its mechanical role, the cerebrospinal fluid protects the brain through its chemical buffering capacity, which provides a rather stable milieu. It also transports nutrients, flushes away waste products, and serves as a medium for the diffusion of neuroendocrine and neurotransmitter substances.

The fluid is produced continuously, at a rate of some 30 mL per hour in the dog, and first circulates through the ventricular system, moved onward by the filtration pressure and ciliary activity of the ependymal lining. It then escapes from the interior of the brain through the lateral apertures of the fourth ventricle (Figure 8–60/7; in some species there is a third median opening unrelated to the plexus). The fluid bathes the brain and cord before returning to the blood, mostly through the arachnoid granulations (villi) (Figure 8–64/10), projections of the arachnoid and subarachnoid space that pierce the dura to enter the dorsal sagittal venous sinus of the brain; these formations become increasingly prominent with age. (Obliteration of the villi results in hydrocephalus because drainage of the fluid is hampered while its production continues and is not influenced by a feedback mechanism.) A smaller part of the fluid percolates along the meningeal cuffs that surround the cranial and spinal nerves at their origins and is eventually absorbed by perineural lymphatics; these connections are believed to provide potential routes for the retrograde (i.e., toward the meninges and nervous tissue) spread of infection.

THE ARTERIAL BLOOD SUPPLY

The blood supply to the brain comes mainly from the circulus arteriosus cerebri (formerly known as the circle of Willis), which lies ventral to the hypothalamus where it forms a ring around—but at some distance from—the infundibular stalk. The appearance of the circle and the pattern of its major branches are remarkably constant among mammals, although the sources from which the circle is supplied and the directions in which blood flows in certain vessels vary. For this reason, the initial account is based on the arrangements in the dog, an animal in which the arrangement is not only relatively simple but also of the most common pattern.

The arterial circle of the dog is supplied from three sources: paired internal carotid arteries laterally and the basilar artery caudally (Figure 8–65). The internal carotid artery (Figure 8–65/5) is a terminal branch of the common carotid from which it springs opposite the pharynx. It then runs toward the base of the skull. In many species the artery makes immediate entry to the cranial cavity through a carotid foramen in the cranial floor, but in the dog it must first traverse a tunnel (carotid canal) in the bone medial to the tympanic bulla. The artery is released at the rostral end of the tunnel and describes a loop that first carries it ventrally, then dorsally, before it finally gains the cranial cavity. It then penetrates the outer meninges, which involves passage through the cavernous venous sinus enclosed within a splitting of the dura, before dividing into divergent branches. The rostral branch unites with its fellow to complete the rostral half of the circle, the half from which the large rostral and middle cerebral arteries arise. The caudal branch anastomoses with a branch of the basilar artery (which reaches the circle along the midventral surface of the brainstem) to complete the circle (Figure 8–65/11). The caudal cerebral and rostral cerebellar arteries leave the caudal half of the circle; the fifth major artery to the brain, the caudal cerebellar, leaves the basilar artery directly.

The blood within the basilar artery has a composite origin. The artery appears to be the direct continuation of the small ventral spinal artery but is greatly reinforced by anastomosis with the vertebral artery (Figure 8–65/13), which passes into the vertebral canal through the atlas. The vertebral artery itself receives anastomotic branches (dog and horse) from the occipital artery (another branch of the common carotid) before entering the canal, and it would thus appear that this vessel (occipital artery) also contributes to the supply of the brain. However, the vertebral artery is the main if not sole supply to the occipital lobes of the cerebral hemispheres and other caudal parts of the brain.

The arrangement is more complicated in many other species. In these the internal carotid connects with other arteries of the head, especially the maxillary, before discharging into the arterial circle. The anastomosis may be small initially, but in many species it later enlarges and detaches many tortuous branches, which together substitute for the original single channel. This arrangement, which may present a rather tangled appearance, is known as a rete mirabile and has a rather enigmatic significance; the arrangement enhances the efficiency of the blood-cooling mechanism that is discussed shortly. In some species the lumen of the part of the internal carotid artery proximal to the rete becomes obliterated, sometimes only a considerable time after birth; when this happens, the emissary artery from the rete delivers blood that is wholly of external carotid origin (see Figure 7–35). This arrangement is found in both sheep and cattle, although these species differ in other features of the arterial supply to the brain (p. 661).

The brain, particularly its gray substance, has very high metabolic requirements, and the arterial supply is commensurate with this, amounting to 15% or 20% of the cardiac output. Despite this, the vessels that actually penetrate the brain are uniformly small, which is a feature that may be related to the need to avoid large, pulsating trunks within the delicate brain tissue. Moreover, in sharp contrast to the wide anastomoses between the feeding vessels, any intracerebral anastomoses are narrow and mostly connect functional end-arteries. This fact, coupled with the very limited regeneration capacity of brain tissue, explains why the most serious consequences may attend occlusion or rupture of a small vessel that may be the sole effective supply to some vital nucleus or tract. Notorious examples are provided by the small arteries within the human corpus striatum, where an infarct is so often the cause of a stroke.

The permeability of the blood capillaries of the nervous tissue is reduced, resulting in the blood–brain barrier. The main structural components of this barrier are provided by the continuity between the endothelial cells of these capillaries and pericytes, astrocytes, and the basement membrane surrounding these capillaries (see p. 217; Figure 8–66).

The spinal cord is supplied by three arteries that run its length. The largest, the ventral spinal artery, follows the surface of the ventral fissure of the cord; paired dorsolateral spinal arteries run close to the furrow from which the dorsal roots of the spinal nerves arise. All three are periodically reinforced by branches from regional arteries: vertebral in the neck and intercostal, lumbar, and sacral in the trunk. These enter at the intervertebral foramina, often in the form of narrow vessels that accompany the roots of the spinal nerves; they form plexuses on the surface of the cord with which the major longitudinal arteries connect. This theoretically regular pattern is subject to much variation, both specific and individual, in which many expected reinforcing arteries are lacking, the plexus is unevenly developed, and stretches of the longitudinal trunks are attenuated.

Branches of the ventral spinal artery supply the “core” of the cord, the gray substance, and the adjacent layer of white substance by an approach through the ventral fissure (see Figure 19–5). The greater part of the white substance is supplied by radial twigs from the dorsolateral arteries and surface plexus. Internal anastomoses between the two sets of vessels, although common, are of questionable efficiency.

THE VENOUS DRAINAGE

A complicated system of venous sinuses within the cranial cavity and vertebral canal is connected at intervals to the exposed regional veins. The cranial sinuses enclosed within the dura mater are divided into dorsal and ventral systems between which there is only limited communication (Figure 8–67). The dorsal system collects blood from the dorsal parts of the brain and the diploë of the bones of the cranial vault. It includes a dorsal sagittal sinus within the falx cerebri. The dorsal sagittal sinus receives numerous tributary veins directly from the cerebral hemispheres, and it is joined toward its caudal end by the straight sinus, which runs within the ventral part of the falx and collects blood from a major vein draining deeper parts of the brain. The dorsal sinus splits (in a variable manner) into bilateral transverse sinuses within the tentorium cerebelli; each later divides—one branch leaving the skull through a foramen, the other connecting with the ventral system.

The ventral or basilar system drains the ventral part of the brain (and other cranial contents and walls) and also receives a major inflow from a vein that enters the cranial cavity from the orbit after draining much of the face, including the nasal cavity. The rostral part of the longitudinal trunk of the ventral system, the cavernous sinus (Figure 8–67/6), is connected with its fellow both before and behind the hypophysis. It divides caudally into the basilar sinus, which continues through the foramen magnum as the main component of the internal vertebral plexus, and a branch that receives a connection from the dorsal system before emerging through a ventral foramen to contribute to the maxillary vein.

The flow of blood into the cranial cavity from the face is noteworthy for two reasons. First, it provides a potential pathway for the spread of infection from the face to the cranial contents. Secondly, it provides for cooling of the arterial supply to the hypothalamus, the part of the brain responsive to and concerned with the regulation of body temperature. The cooling is due to the passage of the internal carotid artery (or rete substitute) through the cavernous sinus, where bathing by venous blood at a somewhat lower temperature (because it is drained from the nose and superficial structures of the head) promotes heat exchange. (An additional mechanism for protecting the brain from damaging hyperthermia is provided by the course of the common carotid artery, which is extensively related to the trachea at no great depth below the skin. These relationships promote heat loss, especially because any physical exertion that tends to raise body temperature also increases the flow of air within the upper respiratory tract.)

The vertebral venous plexus is probably more important clinically. It runs the whole length of the vertebral column and drains blood from the vertebrae, the adjacent musculature, and the structures within the vertebral canal. It gives rise to segmental veins that leave the canal through the intervertebral foramina to join the principal venous channels of the neck and trunk: the vertebral, cranial caval, azygous, and caudal caval veins (see Figure 7–43/18). The major part of the plexus consists of paired channels within the epidural space ventral to the cord. They are composed of crescentic segments that extend between successive intervertebral foramina (see Figure 26–5). The enlarged midpart of each segment swings toward and is generally joined to its neighbor over the middle of the vertebra, which produces a ladderlike pattern of vessels. The connections with segmental veins through the intervertebral foramina form a plexus around the emerging spinal nerves, protecting them from injury.

The veins composing the plexus are thin walled and, being without valves, may pass blood in either direction. They are capacious and adjust in size to compensate for variations in venous return to the heart induced by the intrathoracic pressure changes that accompany respiration. Since the system provides alternative channels to the major systemic veins, it may mitigate the effects of jugular obstruction (when the neck is compressed) or caudal caval obstruction (when pressure within the abdomen is raised). The intermittency of flow caused by these several factors facilitates the spread of septic or neoplastic disease to the vertebral column when the lungs would be the expected destination; blood diverted into the vertebral plexus when the flow through other channels is impeded may be temporarily held stagnant, which allows tumor seeds or microorganisms to settle within the tributaries that issue from the bones.

A further point of clinical importance lies in the risk of hemorrhage when epidural or subarachnoid puncture is performed. The risk is greatest at the atlantooccipital space, where tributaries of the plexus most often encircle the dural tube.

There are no lymphatics in the central nervous tissue.

THE OLFACTORY NERVE (I)

The fibers that compose the olfactory nerve arise as the central processes of the olfactory cells of the nasal mucosa. They are collected into a number of filaments that separately traverse the cribriform plate to join the adjacent surface of the olfactory bulb (Figure 8–19/1). The further course of the olfactory pathways has been described (p. 291).

The short course and deep location protect these nerves against casual injury, and though they may be involved in infectious or neoplastic disease, interference with the sense of smell is more often due to blockage of the air passages leading to the olfactory mucosa. The filaments are surrounded by meningeal sheaths enclosing extensions of the subarachnoid space, which provide potential routes for the spread of infection from the nose to the cranial cavity.

The vomeronasal organ is also part of the olfactory system (see p. 352)

THE OPTIC NERVE (II)

The optic nerve mediates the visual sense and is in fact a brain tract connecting the retina with the diencephalon (from which it originated). The intracranial part of the nerve extends from the optic chiasm (Figure 8–19/7), where varying proportions of the fibers decussate (p. 299), to the optic foramen at the apex of the orbital cone; the intraorbital course is described elsewhere (p. 345) (see Figure 9–17/9). The optic nerve is also enclosed within extensions of the meninges, and the dura blends with the sclera where the nerve joins the eyeball. Section of the nerve obviously results in blindness of that eye.

THE OCULOMOTOR NERVE (III)

The oculomotor nerve consists of somatic efferent fibers from the principal (motor) nucleus and visceral efferent fibers from the parasympathetic nucleus (of Edinger–Westphal), both of which are within the tegmentum of the midbrain (see Figure 8–25). Fibers of the two categories emerge together at the superficial origin from the ventral aspect of the midbrain, close to the midline and in series with other cranial nerves of predominantly somatic efferent composition and with the ventral roots of spinal nerves (Figure 8–19). In its intracranial course the oculomotor is related to the trochlear, abducent, and ophthalmic nerves and to the cavernous sinus, and it passes through the orbital fissure in their company. It divides within the orbit to supply the dorsal, medial, and ventral recti, the ventral oblique, and the levator muscle of the upper eyelid (some authors also include part of the retractor bulbi). The preganglionic parasympathetic fibers synapse within the small ciliary ganglion placed on one of the branches (Figure 8–45/9 and 8–70/1,6). From here, postganglionic fibers pass within the short ciliary nerves to supply the intraocular ciliary and constrictor pupillae muscles. Isolated injury or involvement in disease is not common; the effects can be deduced from consideration of the actions of the muscles it supplies (p. 345).

THE TROCHLEAR NERVE (IV)

The trochlear nerve, which is small, is motor to the dorsal oblique muscle. The nucleus of origin within the tegmentum of the midbrain gives rise to a fiber bundle that decussates internally before emerging from the rostral medullary velum (Figure 8–23/8). The nerve then follows the edge of the tentorium cerebelli to the floor of the cranial cavity. In some species it makes a separate entrance to the orbit, but usually it passes through the orbital fissure. The effects of section are those of paralysis of the dorsal oblique (p. 341).

THE TRIGEMINAL NERVE (V)

The trigeminal nerve, the largest of the cranial nerves, is sensory to the skin and deeper tissues of the face and motor to the muscles of first pharyngeal (mandibular) arch origin. The proprioceptive fibers, which include many from muscles that receive their motor innervation from other cranial nerves, pass to the rostral sensory mesencephalic nucleus; the other afferent fibers pass to the pontine and spinal nuclei. The efferent fibers originate in the motor nucleus (Figure 8–25/7,17). The peripheral nerve is formed by the fusion of sensory and motor roots that attach to the ventrolateral aspect of the pons. The larger sensory root carries the massive trigeminal ganglion and, just beyond this, divides into the trio of primary branches that give the trunk its name. The mandibular branch unites with the motor root to constitute the mixed mandibular nerve; the ophthalmic and maxillary divisions remain purely sensory at this level, although peripheral connections with other cranial nerves introduce somatic and visceral efferent fibers into certain branches. The mandibular nerve emerges through the oval foramen in the floor of the cranial cavity. The ophthalmic and maxillary nerves run rostrally to emerge through the orbital fissure and round foramen, respectively (in ruminants the two openings are combined).

The three primary divisions are initially each restricted to a different process of the embryonic face, which explains the crisply defined adult territories (cf. the dermatomes of the trunk). The ophthalmic nerve supplies the frontonasal process, the primordium of the forehead and nose regions; the maxillary nerve supplies the maxillary process, the primordium of the upper jaw and associated parts; and the mandibular nerve supplies the mandibular process, the primordium of the lower jaw and associated parts, which include the masticatory and other first pharyngeal arch muscles (Figure 8–68).

The ophthalmic nerve (Figure 8–68/1), for which the convenient notation is V-1, divides into three divergent branches soon after entering the orbit. The lacrimal nerve (Figure 8–68/3) passes to the lateral part of the orbital perimeter and, after detaching branches to the lacrimal gland and other deeper structures, emerges to supply the skin about the lateral angle of the eye. The more considerable territory of the frontal nerve (Figure 8–68/2) includes much of the upper eyelid, the forehead, and, through branches that penetrate the bone, the mucosa of the frontal sinus.

The nasociliary nerve (Figure 8–68/4) runs toward the medial wall of the orbit. One branch, the infratrochlear nerve (Figure 8–68/4′), emerges on the face after supplying structures at the medial angle; it supplies another portion of the mucosa of the frontal sinus and in small ruminants detaches the principal nerve to the horn. Other branches of the nasociliary nerve include long ciliary and ethmoidal nerves. The long ciliary nerves (Figure 8–68/4″) penetrate the posterior aspect of the eyeball to supply sensitive tissues, including the cornea; the ethmoidal nerve first re-enters the cranial cavity through the ethmoidal foramen and subsequently passes to the nasal cavity via the cribriform plate before dividing into medial and lateral branches to the mucosa.

The maxillary nerve (V-2) runs across the wall of the pterygopalatine fossa ventral to the orbit (Figure 8–68/5). It bears, or lies close to, the pterygopalatine ganglion, but the relationship is purely topographical. It then enters the infraorbital canal at the maxillary foramen, where it becomes known as the infraorbital nerve (Figure 8–68/6) in anticipation of its reappearance on the face at the infraorbital foramen.

Collateral branches detached within the pterygopalatine fossa include the zygomatic nerve (Figure 8–68/7), which supplies the lower eyelid and adjacent skin and is the origin of the principal nerve of the horn in cattle.

The second branch, the pterygopalatine nerve (Figure 8–68/8), detaches the lesser palatine nerve (Figure 8–68/9) to the soft palate; the greater palatine nerve (Figure 8–68/10), which reaches the hard palate after traversing the palatine canal and supplies both the palatine mucosa and the floor of the nasal vestibule; and the caudal nasal nerve (Figure 8–68/11), which passes through the pterygopalatine foramen to supply mucosa of the ventral part of the nasal cavity, maxillary sinus, and palate.

Within the infraorbital canal the infraorbital nerve (Figure 8–68/6) detaches short twigs to the alveoli of the cheek teeth and nasal mucosa and longer rostral alveolar branches that continue within the bone, beyond the infraorbital foramen, to the alveoli of the canine and incisor teeth. After emerging at the infraorbital foramen the infraorbital nerve supplies various labial and nasal branches to the structures of the muzzle, including some branches that run back over the nose to the edge of the infratrochlear territory. Although covered by muscle at its emergence, the infraorbital nerve may usually be palpated, stimulated by pressure, or blocked by injection of local anesthetic solution.

On leaving the cranium, the mandibular nerve (V-3) detaches in close succession several nerves that pass to the masseter, temporalis, medial and lateral pterygoid, tensor veli palatini, and tensor tympani muscles (Figure 8–68/12). There are minor variations in their pattern, and those to the masseter and temporalis are often initially conjoined in a short masticatory nerve (Figure 8–68/13). The masseteric nerve passes to that muscle between the coronoid and condylar processes of the mandible. The deep temporal nerves (Figure 8–68/14) run dorsomedially to the temporalis. The otic ganglion lies close to the origin of the pterygoid nerves (Figure 8–68/16).

The next branch, the buccal nerve (Figure 8–68/15), is sensory to the tissues of the cheek, which it reaches after first passing between the pterygoideus and temporalis and then between the maxillary tuber and mandible. Its origin is followed by that of the auriculotemporal nerve (Figure 8–68/17), which bends around the caudal border of the mandible to enter the face a little ventral to the temporomandibular joint. It is sensory to the skin of the temporal region and over much of the external ear, including the lining of the canal leading to the eardrum. The continuation onto the face, the transverse facial branch, supplies a strip of skin extending to the corner of the mouth.

The mandibular nerve continues between the medial and lateral pterygoid muscles before dividing into its end-branches, the lingual and inferior alveolar nerves.

The lingual nerve (Figure 8–68/18) detaches twigs to the oropharyngeal mucosa before dividing into a deep branch that enters the tongue and a superficial branch, the sublingual nerve (Figure 8–68/18′), that runs medial to the mylohyoideus below the mucosa of the oral floor that it supplies. The branch to the tongue is joined by the chorda tympani, a branch of the facial, which introduces visceral efferent fibers to salivary glands that synapse in the adjacent mandibular ganglion and gustatory fibers (special visceral afferent) from the taste buds of the rostral two thirds of the tongue. Other sensory fibers supply general sensation in the same area of the lingual mucosa.

The inferior alveolar nerve (Figure 8–68/19) detaches the mylohyoid nerve (Figure 8–68/19′) for the mylohyoideus and rostral belly of the digastricus before entering the mandibular canal at the mandibular foramen. The inferior alveolar nerve supplies the lower cheek teeth before a large part reappears at the mental foramen as the mental nerve (Figure 8–68/19″), which supplies tissues of the lower lip and chin. In some species several mental branches exit through as many foramina. Although also covered by muscle, the mental nerve(s) can be palpated, compressed, and blocked at emergence.

Injuries to, or disease of, the branches of the trigeminal nerve produce sensory deficiencies in their territories and sometimes manifest themselves by chronic facial irritation; some branches are frequently blocked for minor surgery of the head. Destructive lesions of the mandibular nerve produce paralysis of the muscles that raise the jaw; when the lesion is unilateral, the resulting atrophy may be more obvious than any motor disability. A temporary idiopathic bilateral paralysis of the trigeminal musculature, characterized by a dropped jaw, has been reported in dogs.

THE ABDUCENT NERVE (VI)

The fibers of the abducent nerve originate within the caudal brainstem and make the expected appearance (for general somatic efferent fibers) close to the midline (Figure 8–19). The intracranial course leads to the orbital fissure (or the foramen orbitorotundum); within the orbit the nerve branches to go to the lateral rectus and the retractor, although the exact innervation of the latter muscle is still controversial. Injury produces inability to deviate the eyeball laterally (p. 341).

THE FACIAL NERVE (VII)

The facial nerve is sometimes known as the intermediofacial nerve, a term that indicates its composite nature. The intermediate component is a visceral nerve with sensory (including gustatory) and motor (parasympathetic) functions; the facial component is the nerve of the second pharyngeal arch whose main distribution is to the mimetic musculature.

The facial and vestibulocochlear nerves arise close together at the lateral extremity of the trapezoid body (Figure 8–21/VII,VIII) and run within common meningeal investments to the internal acoustic meatus of the petrous temporal bone. They part here, and the facial nerve enters a passage (facial canal) within the bone that leads, via a sharp caudal convexity (“genu”), to the stylomastoid foramen, where the nerve appears at the surface of the skull. The facial nerve carries the appropriately named geniculate ganglion at the summit of the bend. With the exception of the small branch to the stapedius muscle, the branches detached within the bone represent the intermediate component and those detached after leaving the bone represent the facial component (Figure 8–69/1).

The greater petrosal nerve is detached at the level of the ganglion and emerges through an independent foramen. It is initially parasympathetic but is shortly joined by sympathetic fibers to form a composite autonomic nerve; this, the nerve of the pterygoid canal, runs through that fine passage to reach the pterygopalatine ganglion within the pterygopalatine fossa (Figure 8–70/7,11). The nerve of the pterygoid canal is discussed more fully later (p. 327). The stapedial nerve, which arises next, is motor to the stapedius muscle of the middle ear. The next branch, the chorda tympani (Figure 8–70/13), crosses the tympanic cavity to emerge at the petrotympanic fissure, after which it converges on and becomes incorporated within the lingual branch of the mandibular nerve (p. 353).

The first branches of the free portion of the facial nerve are the internal and caudal auricular nerves, which supply muscles of the external ear and other branches to some hyoid muscles, including the caudal belly of the digastricus. The main trunk enters the face by turning around the mandible, where it is first contained between the masseter and the parotid gland. It divides at about this level (although there are species differences) into three terminal branches.

In some species the auriculopalpebral nerve (Figure 8–69/2) is detached before the main trunk reaches the face, and it is then less vulnerable to injury from superficial trauma to the side of the head. It crosses the zygomatic arch, heading for the space between the upper eyelid and external ear, before dividing into branches that supply the muscles of the eyelids (excluding levator palpebrae superioris) and the auricular muscles in front of the external ear.

The dorsal buccal branch (Figure 8–69/3), which may take the form of a leash of divergent branches, crosses the masseter en route to the muzzle.

In some species the ventral buccal branch (Figure 8–69/4) may take a similar path at a slightly more ventral level, but in others it takes a divergent course, first running within the intermandibular space before entering the face with the parotid duct and facial vessels, where they cross the mandible in front of the masseter. Together, the buccal branches supply the muscles of the cheek, lips, and nostrils. Their peripheral branches join with those of the trigeminal nerve at various levels, and many of the smaller trunks combine motor (facial) and sensory (trigeminal) fibers.

The effects of injury or disease clearly depend on the site of the lesion. Lesions that are situated more centrally, which tend to have more sinister origins, affect the whole facial field and lead to loss of secretory activity by the lacrimal and salivary (except the parotid) glands in addition to muscular paralysis. Lesions involving the main trunk near its exit from the bone paralyze the entire mimetic musculature, while more peripheral lesions may spare some groups, depending on their site and specific and individual variations in the branching pattern. Those confined to the auriculopalpebral nerve produce drooping of the external ear and narrowing of the palpebral fissure with inability to close the eye. Damage to the buccal branches may paralyze the muscles of the lips and cheeks, allowing a quid of food to collect in the oral vestibule. It may also lead to deformation of the muzzle, which is drawn out of symmetry by the unopposed activity of the muscles on the sound side. The alteration in appearance is not always very striking, and the uninjured side, to which the muzzle is drawn, may sometimes appear to have the more distorted aspect. The distortion tends to be more pronounced in the horse and sheep than in other domestic species. (It is important to be aware that in unilateral facial spasm, seen occasionally in the dog, the nose may be drawn toward the affected side.)

The auriculopalpebral nerve is sometimes blocked to facilitate examination of the eye.

THE VESTIBULOCOCHLEAR NERVE (VIII)

The vestibulocochlear nerve divides at the internal acoustic meatus into its vestibular and cochlear parts, which make their separate ways through the petrous temporal bone to the vestibular and cochlear components of the membranous labyrinth of the inner ear. They are discussed further with the special sense organs of balance and hearing (p. 351).

THE GLOSSOPHARYNGEAL NERVE (IX)

The glossopharyngeal nerve combines fibers concerned with the innervation of structures of third pharyngeal arch origin with important visceral efferent (parasympathetic) and afferent components. It is motor to part of the palatopharyngeal musculature and to certain salivary glands and sensory to mucosa of the root of the tongue, palate, and pharynx. In addition, there is an important branch to the carotid sinus and body.

The glossopharyngeal nerve arises from the ventrolateral aspect of the medulla oblongata, from the most rostral rootlets of the linear series that also gives origin to the vagus and the medullary part of the accessory nerve (Figures 8–19 and 8–21). It runs with these nerves to the jugular foramen and at about this level bears two small and rather indistinct ganglia. The first branch, the tympanic nerve, enters the tympanic cavity, where it participates with branches of the facial and internal carotid (sympathetic) nerves in forming a plexus from which a nerve leads to the otic ganglion for the supply of the parotid gland (Figure 8–70/3,14).

The main trunk cleaves for a spell to the vagus and accessory nerves and at this level detaches the carotid sinus branch, which proceeds to the carotid sinus, where it terminates in baroreceptors within the sinus wall and chemoreceptors of the carotid body. The glossopharyngeal nerve then turns rostroventrally, parallel to the stylohyoid, before dividing into pharyngeal and lingual branches. The pharyngeal branches include one to the stylopharyngeus caudalis; the others become diffused within the pharyngeal plexus to which the vagus also contributes. Although most fibers are sensory to the mucosa, the possibility of a further contribution to the pharyngeal musculature seems likely.

The larger lingual branch enters the tongue parallel to the lingual artery, the lingual branch of the mandibular nerve, and the hypoglossal nerve. It is sensory to the mucosa of the root of the tongue (including the taste buds in this area) and motor to the levator palatini muscle and the glands of the soft palate. Damage to the nerve, which is most common in horses as the result of inflammation of the guttural pouch, may lead to difficulties in swallowing. Because the vagus may also be affected, it is difficult to know the extent to which the paresis of palate and pharynx is due to glossopharyngeal involvement. Experimental studies suggest that the role of the glossopharyngeal nerve is more important than many authors have claimed.

THE VAGUS NERVE (X)

The vagus nerve is the nerve of the fourth and subsequent pharyngeal arches. It also contains the parasympathetic fibers that innervate the cervical, thoracic, and abdominal viscera. The second component gives it by far the most widespread distribution of any cranial nerve (Figure 8–73/5).

The vagus forms part of the bundle of nerves that passes through the jugular foramen. It bears two small ganglia on the stretch that lies within and immediately external to the foramen and beyond this runs in close association with the glossopharyngeal and accessory nerves. After the glossopharyngeal nerve turns rostrally, the vagus continues with the accessory and later acquires a relationship with the cranial cervical ganglion. It then continues down the neck in close contact with the sympathetic trunk, with which it is bound within a common fascial sheath, on the dorsal margin of the common carotid artery and related to the trachea. The left vagosympathetic trunk has an additional contact with the esophagus (Figure 8–75/5). The vagus and sympathetic nerves part at the entrance to the chest, after which the vagus continues more or less horizontally through the mediastinum until it divides over the pericardium into dorsal and ventral branches. These combine with the corresponding contralateral branches to form the dorsal and ventral vagal trunks that enter the abdomen along the corresponding borders of the esophagus. Within the abdomen the two nerves branch freely, participating with the sympathetic fibers in forming the plexuses from which the abdominal viscera are supplied (p. 327).

The first significant detachment after the nerve leaves the skull is an auricular branch that takes part in the innervation of the skin of the external ear. This is followed by pharyngeal branches that combine with those of the glossopharyngeal, cranial laryngeal, and sympathetic nerves in forming the pharyngeal plexus. An extension of the plexus supplies the cervical esophagus. The cranial laryngeal nerve goes to the larynx, where it divides into an external branch for the cricothyroid muscle and an internal branch for the laryngeal mucosa from the aditus to the glottis. This branch makes connections with the recurrent laryngeal nerve. The depressor nerve to the heart is formed partly of fibers from the cranial laryngeal nerve and partly of fibers from the main vagal nerve; it is difficult to follow because in most animals it rejoins the main trunk for its further progress through the neck and thorax to the heart.

The thoracic portion of the vagus detaches cardiac branches that form a mediastinal plexus with sympathetic fibers with the same destination. A large caudal (recurrent) laryngeal nerve is also detached within the thorax. That of the right side changes direction by winding around a branch of the subclavian artery, while the left one winds around the aorta. The recurrent laryngeal nerve reascends the neck ventral to the common carotid artery in a course that leads it back to the larynx, where it supplies the bulk of the intrinsic laryngeal musculature (all but the cricothyroideus) and the mucosa caudal to the glottis. Small twigs detached en route pass to the cardiac plexus and to the trachea and esophagus. The distribution of the main trunk is completed by pulmonary branches that combine in a common plexus with sympathetic nerves.

Damage to the vagus nerve and its branches may be manifested in a variety of ways, including difficulties in swallowing and altered functioning of the heart and the other viscera. Degeneration of the recurrent laryngeal nerve is especially common in horses, producing the condition known as roaring (p. 526); it also occurs in dogs.

THE ACCESSORY NERVE (XI)

The accessory nerve is curiously formed of two roots. The spinal root is provided by filaments that emerge midway between the dorsal and ventral roots of the first five (or so) spinal nerves (Figures 8–19 and 8–21). These combine in a trunk that runs within the spinal subarachnoid space to enter the skull through the foramen magnum; it then approaches the cranial root, which is formed by the most caudal rootlets of the glossopharyngeal–vagus series. There is only brief contact between the two roots, and although some fibers may be exchanged, the cranial root then amalgamates with the vagus to which it probably furnishes the fibers that reach the laryngeal musculature via the recurrent laryngeal nerve. It is the spinal root that forms the accessory nerve of descriptive anatomy. This passes through the jugular foramen to divide within the atlantal fossa into dorsal and ventral branches.

The dorsal branch runs caudally over the splenius and serratus ventralis before it supplies the covering brachiocephalicus, omotransversarius, and trapezius. The ventral branch supplies only one muscle, the sternocephalicus, which it enters close to its cranial attachment.

There is no convincing explanation for the curious detour made by the spinal fibers of this nerve.

THE HYPOGLOSSAL NERVE (XII)

The hypoglossal nerve is motor to the intrinsic and extrinsic muscles of the tongue, which originate in the myotomes of occipital somites. After leaving the ventral aspect of the medulla oblongata (Figure 8–19), the nerve passes through the hypoglossal canal before crossing the nerves of the vagus group to continue toward the tongue, which it enters ventral to the glossopharyngeal nerve. It ramifies within the tongue substance to reach the various muscles.

A destructive lesion of this nerve paralyzes the ipsilateral muscles, allowing a deviation of the tongue toward the normal side. A marked atrophy eventually develops.

THE DORSAL RAMI

As a rule, the dorsal rami are considerably smaller than the ventral and have simpler distributions. Each divides into a medial branch that supplies the local part of the epaxial musculature of the neck, trunk, or tail and a lateral branch that is distributed to the dorsal part of the skin segment (dermatome) served by the particular spinal nerve. These areas extend from the dorsal midline for a variable distance over the side of the animal. The territories of the first few cervical nerves extend onto the poll region in addition to supplying skin over the neck, those of the nerves to each side of the cervicothoracic junction supply skin over the upper part of the shoulder, and those of the middle and caudal thoracic and lumbar regions serve increasingly larger areas of the skin of the chest wall and flank; however, those of the sacral nerves are again restricted. Inconspicuous connections between neighboring nerves form a continuous plexus through which exchange of fibers blurs the boundaries between the areas supplied by individual nerves; indeed, it is probable that every part of the skin receives sensory fibers from two, if not three, spinal nerves.

THE VENTRAL RAMI

The larger ventral rami supply the hypaxial muscles, including those of the limbs (excepting the thoracic girdle muscles supplied by the eleventh cranial nerve and the rhomboideus supplied in some species by dorsal rami) and the remaining skin of the neck, trunk, and limbs. Except in the thoracic region, where a more precise segmental distribution is retained, the ventral rami are also joined with their neighbors by connecting branches. These connections are greatly exaggerated at the levels of origin of the nerves to the forelimb and hindlimb, where they constitute the brachial and lumbosacral plexuses, respectively.

THE PARASYMPATHETIC SYSTEM

The preganglionic cells of the parasympathetic system are restricted to a number of discrete nuclei within the brainstem and to the lateral column of a short stretch of the spinal cord, generally the second, third, and possibly fourth sacral segments (see Figure 8–73). The aptly designated craniosacral outflow is confined to the oculomotor, facial, glossopharyngeal, vagus, and pelvic nerves.

The cranial parasympathetic pathways have rather limited anatomical independence. Varying parts of their courses are incorporated within nerves of predominantly somatic composition. Exclusively parasympathetic bundles are found only close to the target organs. The chief, grossly visible features of the cranial parasympathetic outflow have been described with the relevant nerves, and it now remains to draw these threads together.

The most rostral parasympathetic nucleus, the parasympathetic oculomotor nucleus, lies within the midbrain in association with the motor nucleus of the third cranial nerve. The parasympathetic preganglionic fibers emerge from the main trunk within the orbit to constitute the oculomotor (short) root of the ciliary ganglion. Beyond the ganglion, the postganglionic fibers proceed as the short ciliary nerves, which also incorporate sympathetic and sensory fibers; these nerves penetrate the sclera to form the ciliary plexus from which the parasympathetic fibers extend to the ciliary and pupillary sphincter muscles (Figure 8–70/6,10).

The parasympathetic component of the facial nerve originates in the rostral parasympathetic (salivatory) nucleus of the medulla oblongata (Figure 8–70/2). The preganglionic fibers are incorporated within the main facial trunk, run through the somatic geniculate ganglion without interruption, and later leave in the chorda tympani and the greater petrosal nerve (Figure 8–70/11,13). The chorda tympani introduces its complement to the lingual nerve from which the parasympathetic fibers later emerge to synapse within the mandibular ganglion; the postganglionic fibers supply the mandibular and sublingual salivary glands.

The greater petrosal nerve is joined by the deep petrosal (sympathetic) nerve (Figure 8–70/12) to constitute the nerve of the pterygoid canal, which leads to the pterygopalatine ganglion (Figure 8–70/7). The postganglionic fibers join the lacrimal nerve (after passage through the zygomatic nerve) en route to the lacrimal gland and various other branches of the maxillary nerve en route to glands within the nasal and palatine mucosae.

The parasympathetic component of the glossopharyngeal nerve originates from the middle parasympathetic nucleus in the medulla oblongata (Figure 8–70/3). The preganglionic fibers pass through the somatic ganglion of this nerve before joining the tympanic plexus; from this they proceed to the otic ganglion (Figure 8–70/9). The postganglionic fibers are carried via the pterygoid nerve and a communicating branch of the auriculotemporal nerve to the parotid gland.

The parasympathetic component supplies the bulk of the vagus nerve; indeed it is the whole complement distal to the origin of the recurrent laryngeal nerve (Figure 8–73/5,6). The preganglionic fibers proceed to numerous small ganglia scattered along the nerve plexuses that supply and are often located within the tissues of the target organs. The plexuses include the cardiac and pulmonary plexuses within the chest (Figure 8–73/7) and the gastric, hepatic, mesenteric, gonadal, and renal plexuses formed by the confluence of branches of the vagal trunks with sympathetic nerves within the abdomen (Figure 8–73/10). Broadly, the dorsal vagal trunk supplies hepatic and gastric plexuses, and the larger ventral vagal trunk supplies celiac, mesenteric, renal, and gonadal plexuses.

The fibers of the sacral parasympathetic outflow are initially incorporated in certain sacral ventral rami from which they emerge to constitute the pelvic nerves (Figure 8–73/11). These form a retroperitoneal plexus that is joined by sympathetic fibers delivered by the hypogastric nerves that descend from the caudal mesenteric ganglion. Numerous minute ganglia are found scattered in the plexus, whereas other (terminal) ganglia are embedded within the walls of predominantly pelvic viscera: the descending colon, rectum, bladder, uterus, and vagina (in the female); accessory reproductive glands (in the male); and the genital erectile tissue. The parasympathetic pathways have their peripheral synapses exclusively in the terminal ganglia, whereas some sympathetic peripheral synapses are divided among the plexus and terminal ganglia.

THE SYMPATHETIC SYSTEM

The preganglionic fibers of the sympathetic system take their origin from the lateral column of the thoracolumbar part of the spinal cord (Figure 8–74/1) and pass into the ventral roots of the thoracic and first several lumbar nerves. They continue into the spinal nerves and then issue from the ventral rami, constituting the white communicating branches, which join the ganglia of the sympathetic trunks (Figure 8–53/5,7). The bilateral trunks run the length of the neck and back, and each have a basically segmental arrangement, although strict correspondence of the ganglia with spinal nerves is evident only in the thoracic and cranial lumbar regions.

The cervical part of the trunk begins at the large, spindle-shaped, cranial cervical ganglion placed close to the base of the skull (Figure 8–74/5). The trunk is associated with the vagus within the carotid sheath and forms the vagosympathetic trunk that proceeds down the neck. The two components part company at the entrance to the chest, where the sympathetic trunk often bears a middle cervical ganglion by the first rib (Figure 8–75/7′). The trunk then continues subpleurally, over the line of the costovertebral articulations, before passing dorsal to the diaphragm to gain admission to the abdomen. Its thoracic part shows a regular arrangement of ganglia, although the first one or two are fused with a caudal cervical element to form the large cervicothoracic ganglion deep to the head of the first rib (Figure 8–75/7). The lumbar part of the trunk, which lies between the psoas musculature and vertebral bodies, at first also carries a regular complement of ganglia, but the arrangement later becomes more erratic in that some caudal lumbar ganglia split into two or, less commonly, fuse with their neighbors. The sacral part is even less regular and may fuse, temporarily or finally, with its fellow before extending into the tail, where it rapidly fades (Figure 8–74/3).

Because the sympathetic outflow is restricted, it follows that only the thoracic and cranial lumbar ganglia are joined by white communicating branches. However, all spinal and many cranial nerves are joined by bundles (gray communicating branches) of postganglionic fibers destined for vessels, skin glands, and so forth. It should be stressed that the body wall and limbs are innervated only by these postganglionic sympathetic fibers. Those to most cervical nerves join within a single trunk, the vertebral nerve, which runs from the cervicothoracic ganglion through the foramina of successive cervical transverse processes (Figure 8–74/7). The postganglionic sympathetic fibers to the first two cervical nerves and to cranial nerves extend from the cranial cervical ganglion; many form the internal carotid nerve that follows the like-named artery.

Several alternative fates are open to the preganglionic fibers that enter the sympathetic chain, each to project on many ganglion cells. Some fibers synapse immediately within the local ganglion, others run cranially or caudally within the trunk to synapse within ganglia that are more cranial or caudal in the series, and yet others pass uninterruptedly through the trunk to proceed to a second set of (prevertebral) ganglia placed about the origin of the visceral branches of the abdominal aorta (Figures 8–74/10,11 and 8–75/11,12). This last group constitutes the splanchnic nerves, which are rather variable in arrangement; usually one greater splanchnic nerve is formed by preganglionic fibers that leave the trunk from about the sixth to the penultimate thoracic ganglia with lesser thoracic and lumbar splanchnic nerves arising at more caudal levels (Figure 8–74/8,9).

The viscera and vessels of the head receive their sympathetic innervation via the cranial cervical ganglion. The postganglionic fibers that emerge from this ganglion radiate in a number of directions that carry them into the territories of the cranial and first two cervical nerves. Though many pass through parasympathetic ganglia, they of course do so without interruption. The details are of rather limited clinical importance (although relevant to experimental work), and only a few points are presented here (see Figure 8–70).

One large group of fibers follows the internal carotid artery into the cranial cavity and there provides twigs to the intracranial vessels and fiber bundles that join various nerves, especially the trigeminal and those to the extraocular muscles. Another group of fibers passes through the ciliary ganglion to the eyeball for ultimate distribution to the dilator pupillae. At a more proximal level, the internal carotid nerve gives off the deep petrosal nerve, which combines with the greater petrosal nerve (Figure 8–70/11) in its passage through the pterygoid canal to the pterygopalatine ganglion (Figure 8–70/7). These fibers are ultimately dispersed with the various nerves that supply structures within the orbit, nasal cavity, sinuses, and palate.

Other branches concur with parasympathetic fibers in forming a plexus within the tympanic cavity from which the parotid gland is supplied after passage beyond the otic ganglion. Yet other bundles of fibers entwine the external carotid artery and its branches.

The thoracic organs—heart, trachea, and lungs—are supplied by postganglionic fibers that form cardiac and pulmonary plexuses within the mediastinum after leaving the thoracic portion of the sympathetic trunk. These plexuses combine with the corresponding parasympathetic component (see Figure 8–75).

The abdominal and pelvic organs receive their sympathetic innervation through the various splanchnic nerves that lead to the celiac, cranial mesenteric, renal, aorticorenal, gonadal, and caudal mesenteric ganglia placed on the ventral face of the aorta by the origins of the visceral arteries. The preganglionic fibers synapse in these ganglia and in the postganglionic fibers that emerge from intricate plexuses (combining vagal contributions) that enmesh, and run parallel to, the visceral arteries from which they obtain their names (Figure 8–76).

The pelvic organs are supplied with postganglionic fibers that leave the caudal mesenteric ganglion within the paired hypogastric nerves (Figure 8–76/8). These enter the pelvic cavity below the peritoneum to form a common pelvic plexus with the parasympathetic pelvic nerves (Figure 8–75). As already mentioned, the sympathetic contribution to the pelvic plexus includes preganglionic fibers that have deferred their synapses to peripheral locations within the pelvis.

SUMMARY OF AUTONOMIC INNERVATION

Certain effects of the autonomic nervous system are tabulated (Table 8–3) by way of illustration, but for more controversial points, such as the innervation of the bladder and urethra, or those requiring more detailed description, the reader is referred to modern works of physiology.