Head and Neck

Dural Venous Sinuses

The dura also separates to form several large endothelial-lined venous channels between its periosteal and meningeal layers—superior and inferior sagittal, straight, confluence of sinuses, transverse, sigmoid, and cavernous sinuses—and several smaller dural sinuses (Table 8-2 and Fig. 8-7). These dural venous sinuses drain blood from the brain, largely posteriorly, then into the internal jugular veins. These sinuses lack valves, however, so the direction of blood flow through the sinuses is pressure dependent. Of particular importance is the cavernous venous sinus, which lies on either side of the sella turcica and has an anatomical relationship with the internal carotid artery and several cranial nerves, including III, IV, V₁, V₂, and VI. Injury or inflammation in this region can affect all these important structures. Also, the optic chiasm lies just above this area, so CN II may be involved in any superior expansion of the cavernous sinus (e.g., pituitary tumor).

Subarachnoid Space

The subarachnoid space (between the arachnoid and pia mater) contains cerebrospinal fluid (CSF), which performs the following functions (Figs. 8-5 and 8-8):

The arachnoid granulations absorb most of the CSF and deliver it to the dural venous sinuses (see Figs. 8-5 and 8-8). These granulations are composed of convoluted aggregations of arachnoid that extend as “tufts” into the superior sagittal sinus and function as one-way valves for the clearance of CSF; the CSF crosses into the venous sinus, but venous blood cannot enter the subarachnoid space. Small, microscopic arachnoid cell herniations also occur along the spinal cord, where CSF (which circulates at a higher pressure than venous blood) is delivered directly into small spinal cord veins.

Clinical Focus 8-5   Meningitis

Meningitis is a serious condition defined as an inflammation of the arachnoid and pia mater. It results most often from bacterial or aseptic causes. Aseptic causes include viral infections, drug reactions, and systemic diseases. Patients with meningitis usually present with the following symptoms:

• Headache

• Fever

• Seizures

• Painful stiff neck

Diagnosis is made by performing a lumbar puncture and examining the CSF.

Gross Anatomy of the Brain

The most notable feature of the human brain is its large cerebral hemispheres (Fig. 8-9). Several circumscribed regions of the cerebral cortex are associated with specific functions, and key surface landmarks of the typical human cerebrum are used to divide the brain into lobes: four or five, depending on classification, with the fifth either the insula or the limbic lobe. The lobes and their functions are as follows:

Other key areas of the brain include the following components (Fig. 8-9):

Internally, the brain contains four ventricles, two lateral ventricles, and a central third and fourth ventricle (Fig. 8-10). Cerebrospinal fluid, produced by the choroid plexus (see Fig. 8-5), circulates through these ventricles and then enters the subarachnoid space through two lateral apertures (foramina of Luschka) or a median aperture (foramen of Magendie) in the fourth ventricle.

Blood Supply to the Brain

Arteries supplying the brain arise largely from the following two pairs of arteries (Fig. 8-11 and Table 8-3):

The vertebral arteries give rise to the anterior and posterior spinal arteries (a portion of the supply to the spinal cord) and the posterior inferior cerebellar arteries, and then join at about the level of the junction between the medulla and pons to form the basilar artery (Fig. 8-11). The internal carotid arteries each give rise to an ophthalmic artery, a posterior communicating artery, a middle cerebral artery, and an anterior cerebral artery. Table 8-3 summarizes the brain regions supplied by these vessels and their major branches.

Clinical Focus 8-8   Subdural Hematomas

Subdural hematomas are usually caused by an acute venous hemorrhage of the cortical bridging veins draining cortical blood into the superior sagittal sinus. Half are associated with skull fractures. In a subdural hematoma the blood collects between the meningeal dura and the arachnoid mater. Clinical signs include a decreasing level of consciousness, ipsilateral pupillary dilation, headache, and contralateral hemiparesis. These hematomas may develop within 1 week after injury but often present with clinical signs within hours. Chronic subdural hematomas are most common in elderly persons and alcoholic patients who have some brain atrophy, which increases the space traversed by the bridging veins and renders the stretched vein susceptible to tearing.

Clinical Focus 8-9   Transient Ischemic Attack

A transient ischemic attack is a temporary interruption of focal brain circulation that results in a neurologic deficit that lasts less than 24 hours, usually 15 minutes to 1 hour. The most common cause of TIA is embolic disease from the heart, carotid, or cerebral vessels, which may temporarily block a vessel. The onset of the deficit is abrupt, and recovery is gradual. The most common deficits include the following:

• Hemiparesis

• Hemisensory loss

• Aphasia

• Confusion

• Hemianopia

• Ataxia

• Vertigo

Clinical Focus 8-10   Stroke

Cerebrovascular accident (CVA) or stroke is a localized brain injury caused by a vascular episode that lasts more than 24 hours, whereas a transient ischemic attack (TIA) is a focal ischemic episode lasting less than 24 hours. Stroke is classified into the following two types:

• Ischemic (80%): infarction; thrombotic or embolic, resulting from atherosclerosis of the extracranial (usually carotid) and intracranial arteries or from underlying heart disease.

• Hemorrhagic: occurs when a cerebral vessel weakens and ruptures (subarachnoid or intracerebral hemorrhage), which causes intracranial bleeding, usually affecting a larger brain area.

Clinical Focus 8-11   Carotid–Cavernous Sinus Fistula

More common than symptomatic intracavernous sinus aneurysms but less common than subarachnoid saccular (berry) aneurysms, carotid–cavernous sinus fistulas often result from trauma and are more common in men. These high-pressure (arterial) low-flow lesions are characterized by an orbital bruit, exophthalmos, chemosis, and extra-ocular muscle palsy involving CN III, IV, and VI. Blood collecting in the cavernous sinus drains by several venous pathways because the sinus has connections with other dural venous sinuses as well as with the ophthalmic veins and pterygoid plexus of veins in the infratemporal region.

Clinical Focus 8-12   Collateral Circulation after Internal Carotid Artery Occlusion

If a major artery such as the internal carotid becomes occluded, extracranial and intracranial (circle of Willis) anastomoses may provide collateral routes of circulation. These routes are more likely to develop when occlusion is gradual, as in atherosclerosis, rather than acute, as in embolic obstruction.

Clinical Focus 8-13   Vascular (Multi-infarct) Dementia

Dementia is an acquired neurologic syndrome that presents with multiple cognitive deficits. By definition, dementia includes short-term memory impairment, behavioral disturbance, and/or difficulties with daily functioning and independence. Dementia can be classified as degenerative, vascular, alcoholic, or human immunodeficiency virus (HIV) related. Vascular dementias are caused by anoxic damage from small infarcts and account for about 15% to 20% of dementia cases. Multi-infarct dementia is associated with heart disease, diabetes mellitus, hypertension, and inflammatory diseases.

Clinical Focus 8-14   Brain Tumors

Clinical signs and symptoms of brain tumors depend on the location and the degree to which intracranial pressure (ICP) is elevated. Slow-growing tumors in relatively silent areas (e.g., frontal lobes) may go undetected and can become quite large before symptoms occur. Small tumors in key brain areas can lead to seizures, hemiparesis, or aphasia. Increased ICP can initiate broader damage by compressing critical brain structures. Early symptoms of increased ICP include malaise, headache, nausea, papilledema, and less often abducent nerve palsy and Parinaud's syndrome. Classic signs of hydrocephalus are loss of upward gaze, downward ocular deviation (“setting sun” syndrome), lid retraction, and light-near dissociation of pupils. Primary tumors include the following:

• Gliomas: arise from astrocytes or oligodendrocytes; glioblastoma multiforme is the most malignant form (astrocytic series).

• Meningiomas: arise from the arachnoid mater and can extend into the brain.

• Pituitary tumors: can expand in the sella turcica and affect CN II, III, IV, V₁, V₂, and VI; about 15% of primary tumors.

• Neuromas: acoustic neuroma, a benign tumor of CN VIII, is a common example; about 7% of primary tumors.

Clinical Focus 8-15   Metastatic Brain Tumors

Metastatic brain tumors are more common than primary brain tumors. Most spread via the bloodstream, with cells seeded between the white matter (fiber tract pathways) and gray matter (cortical neurons). Some tumors metastasize directly from head and neck cancers or through Batson's vertebral venous plexus. Presentation often includes headache (50%), seizures (25%), and elevated intracranial pressure.

Cranial Nerves

See Chapter 1 for an overview of the general organization of the nervous system.

In addition to the 31 pairs of spinal nerves, 12 pairs of cranial nerves arise from the brain. As with the spinal nerves, cranial nerves are part of the peripheral nervous system and are identified both by name and by Roman numerals I to XII (Fig. 8-12). Cranial nerves are somewhat unique and may contain the following multiple functional components:

By convention, each cranial nerve is classified as either general (G) or special (S), and then somatic (S) or visceral (V), and finally as afferent (A) or efferent (E). For example, a cranial nerve that is classified GVE (general visceral efferent) means it contains motor fibers to visceral structures, such as a parasympathetic or sympathetic fiber from the spinal cord.

In general, cranial nerves are described as follows (Table 8-4):

Rather than describe each cranial nerve and all its branches in detail at this time, we will review each nerve anatomically and clinically as we encounter it in the various regions of the head and neck. It may be helpful to refer back to this section each time you are introduced to a new region and its cranial nerve innervation. Autonomic components of the cranial nerves and their autonomic ganglia are summarized in Figure 1-25; the more complex cranial nerves also are summarized at the end of this chapter.

Muscles of Facial Expression

The muscles of facial expression are skeletal muscles that lie in the subcutaneous tissue of the face. They are all innervated by the terminal motor branches of the facial nerve (CN VII), and most originate from the underlying facial skeleton but insert into the skin or facial cartilages (Fig. 8-13). Table 8-5 summarizes several of the major facial muscles, which are derived from the second branchial arch (see Embryology).

Innervation of the facial muscles is by the five terminal branches of CN VII. The facial nerve enters the internal acoustic meatus, passes through the facial canal in the petrous portion of the temporal bone, and then descends to emerge from the stylomastoid foramen. CN VII then passes through the parotid salivary gland and distributes over the face and neck (Fig. 8-14). The five terminal motor (branchial motor) branches are as follows:

The sensory innervation of the face is by the three divisions of the trigeminal nerve (CN V), with some contributions by the cervical plexus. Figure 8-15 lists the specific nerves for each division. All the sensory neurons in CN V reside in the trigeminal (semilunar, gasserian) ganglion. The trigeminal nerve is divided as follows:

The blood supply to the face includes the following vessels (Fig. 8-16):

Eyelids and Lacrimal Apparatus

The eyelids protect the eyeballs and keep the corneas moist. Each eyelid contains a tarsal plate of dense connective tissue; tarsal glands that secrete an oily mixture into the tears; modified sebaceous glands associated with each eyelash; apocrine glands (modified sweat glands); accessory lacrimal glands along the inner surface of the upper eyelid; and in the superior eyelid only, a small slip of smooth muscle (superior tarsal [Müller's] muscle), which attaches to the tarsal plate along with the levator palpebrae superioris muscle (Fig. 8-18). The tears contain albumins, lactoferrin, lysozyme, lipids, metabolites, and electrolytes. The lacrimal glands secrete continuously, and as one blinks, the tears are evenly spread across the conjunctiva and cornea. Tears not only keep the eye surface moist but also possess antimicrobial properties. The lacrimal apparatus includes the following:

The lacrimal glands receive secretomotor parasympathetic fibers from the facial nerve (CN VII) that originate in the superior salivatory nucleus. These preganglionic parasympathetic fibers travel in the greater petrosal nerve and in the nerve of the pterygoid canal (vidian nerve), and the fibers then synapse in the pterygopalatine ganglion. Postganglionic parasympathetic fibers travel through the maxillary nerve (V₂), zygomatic nerve, and lacrimal nerve (V₁) to the lacrimal gland (see Fig. 8-70). Postganglionic sympathetic nerves from the superior cervical ganglion (SCG) jump from the internal carotid plexus in the form of the deep petrosal nerve, join the greater petrosal nerve, and form the nerve of the pterygoid canal. These postganglionic sympathetic fibers then follow the same course up to the lacrimal glands. The sensory innervation of the lacrimal gland is through the ophthalmic division of the trigeminal nerve (via the lacrimal branch).

Muscles

The orbital muscles include six extra-ocular skeletal muscles that move the eyeball and one skeletal muscle that elevates the upper eyelid (Fig. 8-19 and Table 8-6). In addition to the movements of elevation, depression, abduction, and adduction, the superior rectus and superior oblique muscles medially rotate (intorsion) the eyeball, and the inferior rectus and inferior oblique muscles laterally rotate (extorsion) the eyeball. The actions of the extra-ocular muscles detailed in Table 8-6 reflect their “anatomical” actions; because of how the muscles insert into the globe, any single action of the eye often involves multiple muscles contracting at the same time. For example, two muscles elevate the eyeball (superior rectus, inferior oblique), and three muscles abduct the eyeball (lateral rectus, superior oblique, inferior oblique). Clinically, one needs to “isolate” the multiple actions of the muscles so that an individual muscle's action can be assessed (e.g., elevation or depression; see Clinical Focus 8-21).

The levator palpebrae superioris muscle elevates the upper eyelid and, from its inferior surface, has a small amount of smooth muscle (superior tarsal muscle) connecting it to the tarsal plate. This smooth muscle is innervated by postganglionic sympathetic fibers from the superior cervical ganglion. The interruption of this sympathetic pathway can lead to a moderate ptosis, or drooping, of the upper eyelid. Interruption of the innervation of the levator palpebrae superioris from CN III can lead to a significant ptosis.

Nerves in the Orbit

Three cranial nerves innervate the extra-ocular skeletal muscles (Table 8-6), one cranial nerve mediates the special sense of sight (CN II), and one cranial nerve conveys general sensory information from the orbit and eye (CN V₁) (Fig. 8-20). The major branches of the ophthalmic nerve (CN V₁) include the following:

The optic nerve (CN II) is actually a brain tract that conveys sensory information from the retina, via the ganglion cell axons, to the brain (see Fig. 8-12). The optic nerve is covered by the same three dural layers as the rest of the CNS, and the retina is really our “window” into the brain (see Clinical Focus 8-25).

In addition to supplying four of the seven skeletal muscles in the orbit (see Table 8-6), the oculomotor nerve (CN III) also provides parasympathetic fibers, which exhibit the following features (see Fig. 8-68):

Sympathetic innervation to the eyeball is arranged as follows (see Figs. 8-67 and 8-68):

Eyeball (Globe)

The human eyeball measures about 25 mm in diameter, is tethered in the bony orbit by six extraocular muscles that move the globe, and is cushioned by fat that surrounds the posterior two thirds of the globe (Fig. 8-21). The outer fibrous white coat of the eyeball is the sclera and is continuous anteriorly with the transparent cornea. A middle vascular layer called the choroid is continuous anteriorly with the ciliary body, ciliary process, and iris. The inner layer is the optically receptive retina posteriorly and an anterior nonvisual retinal extension that lines the internal surface of the ciliary body and iris (Table 8-7).

The large chamber behind the lens is the vitreous chamber (body) and is filled with a gel-like substance called the vitreous humor, which helps cushion and protect the fragile retina during rapid eye movements (see Fig. 8-21).

The chamber between the cornea and the iris is the anterior chamber; the space between the iris and lens is the posterior chamber. Both chambers are filled with aqueous humor, which is produced by the ciliary body and circulates from the posterior chamber, through the pupil, and into the anterior chamber, where it is absorbed by the trabecular meshwork into the scleral venous sinus (canal of Schlemm) at the angle of the cornea and iris.

Muscles of Mastication

The muscles of mastication provide a coordinated set of movements that facilitate biting and chewing (grinding action of lower jaw). These muscles participate in movements of elevation, retrusion (retraction), and protrusion of the mandible. Embryologically, the muscles are derived from the first branchial arch, and all are innervated by CN V₃ (Fig. 8-23 and Table 8-8).

The temporomandibular joint (TMJ) is the articulation between the condylar process of the mandible and the squamous portion of the temporal bone (mandibular fossa) (Figs. 8-24 and 8-25 and Table 8-9). The TMJ is a modified hinge-type synovial joint. Unlike most synovial joints, the TMJ surfaces are covered with fibrous cartilage rather than hyaline cartilage and the joint cavity is divided by a fibrocartilaginous articular disc.

Parotid Gland

The parotid gland is the largest of the three pairs of salivary glands and occupies the retromandibular space between the mandibular ramus and mastoid process (see Figs. 8-14 and 8-16). It is encased within the parotid sheath, a tough extension of the deep cervical fascia. The parotid duct courses medially across the medial border of the masseter muscle and then dives deeply into the buccal fat pad, piercing the buccinator muscle of the cheek and opening in the mouth just lateral to the second maxillary (upper) molar. As noted previously, the terminal portion of the facial nerve to the face exits the stylomastoid foramen and passes through the parotid gland to distribute to the muscles of facial expression (Fig. 8-14). The parotid gland is innervated by secretomotor parasympathetic fibers from the glossopharyngeal nerve (CN IX), which we will review in the next section (see Fig. 8-71).

Infratemporal Fossa

The wedge-shaped infratemporal fossa is the space inferior to the zygomatic arch, medial to the mandibular ramus and posterior to the maxilla. CN V₃, the largest division of CN V, exits the foramen ovale, which is located in the roof of the fossa, and its branches in this region include the following (Fig. 8-26):

Parasympathetic preganglionic fibers from the glossopharyngeal nerve (CN IX) (inferior salivatory nucleus) run through the middle ear tympanic plexus and lesser petrosal nerve to synapse in the otic ganglion, which is located on the medial aspect of CN V₃ as it exits the foramen ovale (see Figs. 8-69 and 8-71). Secretomotor postganglionic fibers join the auriculotemporal nerve and travel to the parotid gland, which they innervate.

Additionally, parasympathetic preganglionic fibers from CN VII (superior salivatory nucleus) pass through the middle ear and exit through a small fissure (petrotympanic) in the temporal bone as the chorda tympani nerve, to join the lingual branch of CN V₃ and pass to the submandibular ganglion, where the fibers synapse (see Fig. 8-70). Secretomotor postganglionic fibers then innervate the submandibular and sublingual salivary glands.

Vascular Supply

The external carotid artery terminates as the superficial temporal and maxillary arteries (see Fig. 8-16). The superficial temporal artery supplies the scalp and upper face via its transverse facial branch. The maxillary artery supplies the infratemporal region, nasal cavities, palate, and maxillary teeth (Fig. 8-27). For descriptive purposes, the maxillary artery is divided into the following three parts:

Major branches of the maxillary artery include the inferior alveolar and middle meningeal branches from the first (retromandibular) part, branches to the muscles of mastication from the second (pterygoid) part, and the superior alveolar, infra-orbital, greater palatine, and sphenopalatine branches from the third (pterygopalatine) part (Fig. 8-27). The terminal portion of the maxillary artery passes into the pterygopalatine fossa (see Fig. 8-27) to gain access to the nasal cavity and nasopharynx. Here it is joined by the maxillary nerve (CN V₂) and its branches.

The infratemporal fossa is largely drained by veins of the pterygoid plexus (Fig. 8-28), which have extensive anastomoses with dural, ophthalmic, and facial veins. Tributaries from each of the areas supplied by the branches of the maxillary artery ultimately drain into the pterygoid venous plexus and/or its principal anastomotic veins. These veins are valveless, so flow can go in either direction based on gravity and pressure.

External Nose

The upper portion of the external nose is continuous with the forehead (frontal bone) through the nasal bones and laterally by the maxillae. The inferior two thirds of the external nose is cartilaginous and formed by the lateral processes of the septal cartilage, a midline septal cartilage, a major alar cartilage (tip of the nose), and several small, minor alar cartilages (Fig 8-30).

Nasal Cavities

Air entering the nose passes through the following areas (Fig. 8-31):

Bones of the nasal cavity include the following (Fig. 8-32):

Blood Supply and Innervation

Blood supply to the nasal cavities originates from the following major arteries (Fig. 8-33):

Corresponding veins drain the floor, lateral walls, and nasal septum, with most of the venous return passing into the pterygoid plexus of veins (Fig. 8-33). Some venous drainage also passes into the facial vein anteriorly and into the inferior ophthalmic veins superiorly.

The innervation of the nasal cavity includes the following (Fig. 8-34; see also Fig. 8-70):