THE NOSE

The nose* (nasus) in the broad sense comprises the external nose, the paired nasal cavities, and the paranasal sinuses. A case may be argued for also including the nasopharynx.

An external nose such as forms so conspicuous a feature of the human face is hardly to be recognized in the domestic species, in which it is merged within the general contours of the muzzle (Figure 4–1). Its extent is more easily determined on palpation as it more or less corresponds with the part of the muzzle skeleton that is cartilaginous and therefore flexible. It is divided internally into two cavities, the nasal vestibules, each of which is entered through a nostril and leads through a region of constriction to the much larger nasal cavity placed beyond. The form and size of the nostrils, their orientation, and the nature of the surrounding integument all show considerable species differences. The integument around the nostrils is naked and sharply demarcated from the unmodified skin in all domestic species other than the horse. According to its extent, the modified region is variously known as the nasal (carnivores, small ruminants), nasolabial (cattle), or rostral (pigs) plate. The nasal plate may be divided by a median groove or philtrum (Figure 4–1/2). The plate is kept moist in cattle, pigs, and dogs; in the first two species the moisture is derived from closely packed underlying glands, whereas in the dog it is an overflow of the secretion of glands of the nasal mucosa, principally the lateral nasal glands.

The cartilages that support the external nose are variable in form, relative size, and even number. The rostral end of the nasal septum forms the median partition between the right and left vestibules and includes a small bone (os rostrale) in the pig. The free edge of the septum gives attachment to other cartilages that support the dorsal and lateral margins of the nostril and determine the form of the opening. One, the alar cartilage, is especially large in the horse and accounts for the curious comma form of the nostril, which is divided in this species into a ventral part, the so-called true nostril leading to the nasal cavity, and a dorsal part, the false nostril leading to a skin-lined diverticulum occupying the nasoincisive notch (see Figure 18–3). The nostril is round in the pig, but in most other species it is prolonged laterally by a slitlike extension. The form of the nostril may be altered, principally by the lateral “wing” (ala) actively being raised by certain facial muscles or passively when the air flow is increased in strenuous breathing or sniffing. These changes can be very pronounced in the horse, leading to compression and almost complete obliteration of the diverticulum.

The integument is carried some distance into the vestibule, where it meets the nasal mucosa at a sharply defined line near which several ducts may open. In the horse these include the nasolacrimal (tear) duct, whose opening is very evident on inspection of the vestibular floor of the live animal; the opening is less easily found in other species, either because the tissues are less pliant (cattle) or because it is placed more deeply (dog). The much smaller openings of the long ducts of the serous lateral nasal glands also discharge in this area. This arrangement aids humidification of the incoming air because the acceleration of flow at the constriction favors vaporization of tears and other watery discharge.

The two nasal cavities occupy a large part of the face: they extend caudally to the transverse bony septum at the rostral end of the cranial cavity (Figure 4–2). Their size may be gauged from the conformation of the head, but the first impression is apt to be grossly misleading. Several features greatly reduce the extent of the cavities below expectation. Firstly, certain bones bounding the cavity are thickened by air spaces (paranasal sinuses) that communicate with the cavity but do not form part of it. Secondly, the embedded portions of the upper cheek teeth occupy a surprising amount of space, especially in the horse. The potential space is also much reduced by certain very delicate mucosa-covered turbinate bones (conchae) that project into the interior from the dorsal and lateral walls. Finally, the walls are covered by a mucosa locally thickened by vascular plexuses (Figures 4–3, 4–4, and 4–5).

The right and left cavities are divided by the nasal septum, which is largely cartilaginous but ossified in its most caudal part (the perpendicular plate of the ethmoid bone). The septum meets the upper surface of the hard palate, which separates the nasal and mouth cavities, but the details vary greatly between species (see Figure 4–5). In the horse the septum meets the whole length of the hard palate so that each nasal cavity communicates with the pharynx through a separate opening (choana) (see Figure 18–11). In other species (e.g., ox, dog) the caudal part of the septum fails to meet the palate and a single opening is shared by the two sides (Figures 4–4/7 and 25–9).

The conchae, which intrude on the cavity, have a complicated and variable pattern. Classified by topography (and not by morphology), they comprise a caudal system (of ethmoidal conchae) constituting the lateral mass or labyrinth of the ethmoid bone and a rostral (nasal) system in which large dorsal and ventral (and a much smaller middle) conchae predominate (Figures 4–2 and 25–9). The numerous ethmoidal conchae are separated by narrow clefts (ethmoidal meatuses), and their pattern is most complicated in species that place much reliance on the sense of smell (Figure 4–4/5,6). The dorsal and ventral nasal conchae impose the meatal pattern of the middle and more rostral parts of the cavity. They are formed of fragile laminae coiled on themselves in a manner that varies with the species and the location. Rostrally, the lamina does not recurve to meet itself and thus bounds a recess of the nasal cavity; more caudally the coil meets itself or the lateral nasal wall to enclose a space that is part of the paranasal sinus system. The conchae reduce the cavity to a series of clefts or meatuses in an arrangement that may be likened to the letter E in transverse section (see Figure 4–5); in other words, the major conchae define dorsal, middle, and ventral meatuses branching from a common meatus against the septum. The dorsal meatus leads directly to the fundus of the nasal cavity and presents air to the olfactory mucosa. The middle meatus usually gives access to the sinus system. The ventral and common meatuses provide the principal airway leading to the pharynx. The relatively wide space at their junction is the route chosen for passage of an instrument such as a stomach tube.

The nasal mucosa blends with the underlying periosteum and varies in thickness. In some parts it is thin, but elsewhere, and especially ventrally, it is much thickened by the inclusion of cavernous blood spaces that make it a semierectile tissue (Figure 4–5/8). The thickness of the mucosa varies with the degree of vascular congestion; when the vessels are most congested, they greatly impede the air flow, causing the stuffiness associated with a head cold.

Apart from olfaction, the nasal cavity has the important function of modifying the incoming air before it is presented to the lower respiratory passages. The air is warmed by passing over the very vascular mucosa, humidified by the vaporization of the tears and serous nasal secretion, and cleansed by contact with the secretion of numerous scattered mucous glands. These glands spread a carpet of mucus over the nasal mucosa that entraps particles and droplets that come into contact with it. The carpet is moved toward the pharynx by the ciliary action of the lining epithelium and is then swallowed. It is said that in the human species as much as half a liter of mucus is swallowed unconsciously each day.

The paranasal sinuses are diverticula of the nasal cavity that excavate the skull bones (Figure 4–6), largely after birth. The separation of the inner and outer tables of the bones alters the conformation of the head and is especially striking in pigs and cattle (Figures 4–7 and 25–11), in which certain sinuses eventually extend dorsal and even caudal to the cranial cavity. The sinuses retain their connections with the nasal cavity, but because the openings are generally narrow, a relatively slow exchange of air occurs. The narrowness and locations of the openings make them prone to blockage when the mucosa is thickened by inflammation or congestion. Not all the sinuses are of equal clinical importance; the surface projections of those commonly involved in disease are considered in the topographical chapters.

All species have frontal and maxillary systems, neither communicating with its contralateral counterpart. The frontal system consists of one or more spaces within the bones at the border between the nasal and cranial cavities. In most species the various frontal compartments open separately into the ethmoidal meatuses in the nasal fundus, but in the horse the frontal sinus communicates with the nasal cavity indirectly via the caudal maxillary sinus.

The maxillary sinus system occupies the caudolateral part of the upper jaw, above the caudal cheek teeth; in some species it sends extensions, variously described as separate sinuses or as diverticula, into the hard palate, the sphenoid bones, the medial aspect of the orbit, and the ventral concha. In the horse the maxillary sinus is divided into caudal and rostral parts, both connected to the middle nasal meatus. In the dog the cavity communicates freely with that of the nose and is known as the maxillary recess.

The function of the sinuses is obscure: they offer some thermal and mechanical protection to the orbit and nasal and cranial cavities, enlarge the skull areas available for muscular attachment without unduly increasing weight, and affect the resonance of the voice.

THE LARYNX

The larynx forms the connection between the pharynx and the tracheobronchial tree. It lies below the pharynx and behind the mouth, suspended from the cranial base by the hyoid apparatus; in most species it is partly contained between the rami of the mandible and partly extended into the neck, where its cartilaginous skeleton is easily recognized on palpation of the living animal (Figure 4–8). Because of its connection with the tongue and hyoid apparatus, the larynx shifts its position when the animal swallows.

THE TRACHEA

The trachea and bronchi form a continuous system of tubes conducting air between the larynx and the smaller passages (bronchioli) in the lungs. They have a very similar construction and together are sometimes termed the tracheobronchial tree.

The trachea leads from the larynx through the visceral space of the neck, enters the mediastinum at the thoracic inlet, and continues to its terminal bifurcation above the heart. The two chief bronchi diverge from the line of the trachea to enter the corresponding lungs at their roots. In ruminants and pigs a separate tracheal bronchus arises proximal to the tracheal bifurcation and separately aerates the cranial lobe of the right lung. The cervical part of the trachea maintains a more or less median position, although its relationship to the esophagus alters at different levels and in different postures of the head and neck (see Figures 3–29 and 4–16/1). Other relations in the neck include the ventral strap muscles of the neck and the carotid sheath and its contents; the common carotid artery commences ventrolaterally but gradually climbs to a dorsolateral position where the trachea originates from the larynx.

The thoracic part of the trachea is deflected slightly to the right where it crosses the aortic arch. It is related ventrally to the cranial vena cava, to the arteries arising from the aortic arch, and to various tributaries and branches of these vessels; it is related dorsally to the esophagus and related variously to mediastinal lymph nodes. In young subjects it is related to the thymus. The bifurcation lies in the region of the fourth to sixth intercostal spaces but varies with the species and with the respiratory phase.

The chief bronchi very quickly enter the lungs (Figure 4–17), in which they ramify according to a pattern described later (p. 162).

The wall of the trachea is composed of an inner mucosa, a fibrocartilaginous middle layer, and an adventitia (in the neck) or serosa (in the thorax) (Figure 4–18). The mucosa, which continues that lining the infraglottic part of the larynx, may show slight longitudinal folding when the lumen is narrowed. It contains both unicellular and multicellular mucous glands that produce a protective covering of mucus that is continuously moved toward the larynx by the ciliary action of the epithelium. This mucus eventually reaches the pharynx and is swallowed without being noticed. Excessive mucus accumulations may irritate the mucosa, stimulating coughing to clear the airway. The fibrocartilaginous coat is composed of numerous strips of cartilage that are bent to form “rings” that are incomplete dorsally where the ends may fail to meet or may overlap. The edges of the strips are connected to each other by sheets of rather elastic connective tissue continuous with the perichondrium. The ends are joined by the smooth tracheal muscle (Figure 4–18/4), which bridges the gap within the “ring” in most species but is placed externally in the dog and the cat.

The construction of the trachea prevents it from collapsing and allows it to make the necessary adjustment in length when the neck is extended and also when the diaphragm contracts. It is attached to the diaphragm indirectly by the pulmonary ligaments and mediastinal connective tissue and also, more effectively, by the negative intrapleural pressure that couples the lungs to the chest wall, including the diaphragm. Variations in diameter are regulated by the tracheal muscle. In addition to these functional changes, there are permanent species and regional variations in the cross-sectional form and area of the trachea.

The structure of the larger bronchi is identical to that of the trachea if allowance is made for the mergence of their outer surfaces with the peribronchial connective tissue (and through this with the stroma of the lung). On the smaller bronchi the cartilage rings are gradually replaced by irregular plaques, and it is the shedding of the last of these that defines the bronchobronchiolar transition.

Variations in the diameter of the bronchi and bronchioli are relatively greater and more significant than those of the trachea.

Before proceeding, one may need to reread the section on the shape and function of the thoracic cavity (p. 52).

THE PLEURA

Each lung is invested by a serous membrane, the pleura, which also lines the corresponding “half” of the thoracic cavity. Thus, two pleural membranes exist, each arranged as a closed invaginated sac. The space between the right and left sacs forms the mediastinum, a more or less median partition in the thorax within which the heart and other thoracic organs are situated (Figure 4–19/7).

The part of the pleura that clothes the lung directly is known as the visceral or pulmonary pleura (Figure 4–19/4). It is reflected around, and also behind, the root of the lung to become continuous with the mediastinal pleura which, in turn, is continuous with the costal and diaphragmatic pleura; these last three parts are together termed the parietal pleura.

In the healthy animal the pleural cavity is a potential rather than an actual space, and it contains only a small amount (a few milliliters) of serous fluid, which is thinly spread over the pleural surface and facilitates the smooth movement of the lung against the chest wall and of one lung lobe against another. The pressure within the pleural cavity, which is about –5 cm H₂O in the neutral resting position of the chest, represents the difference between the forces that tend to recoil the lung and those that tend to expand the chest. The pressure is not uniform throughout the pleural cavity, and in addition to the expected dorsoventral gradient, local and partly unexplained differences exist; these variations in intrapleural pressure account for regional differences in the expansion and aeration of the lungs. The prevailing negative pressure explains why a surgical or traumatic opening in the chest wall causes an inrush of air into the pleural cavity, collapsing the lung and producing the condition known as pneumothorax.

The pleural sac is always more extensive than the lung, and in certain regions, facing surfaces of parietal pleura are directly applied to each other. The most important example of such an arrangement is found caudal to the basal border of the lung, where the peripheral part of the diaphragmatic pleura rests against the costal pleura lining the chest wall (the costodiaphragmatic recess; Figure 4–19/10). Although the extent of the recess varies with the phase of respiration, it remains considerable even in full inspiration, and the potential of this portion of the pleural sac is therefore never realized (see Figure 4–22/6). A similar but smaller costomediastinal recess is present ventral to the lung (Figure 4–20/12).

Cranially, the costal and mediastinal portions of the pleura come together to form a dome, the cupula pleurae, which may extend in front of the first rib, where it is obviously vulnerable to injury (Figure 4–21/8′). The mediastinum is not symmetrical but is deflected to the left at certain levels. The important deflection of the caudal mediastinum is produced by the greater size of the base of the right lung.

A special fold (plica venae cavae) of the pleura of the right sac extends between the diaphragm and pericardium and carries the caudal vena cava in its free dorsal border (Figure 4–20/3,9). This triangular partition helps define a recess into which the accessory lobe of the right lung fits (Figure 4–21).

Considerable practical significance attaches to the strength of the mediastinum, which varies much between species. In some, for example, the ruminants, the mediastinum is thick and able to withstand a considerable pressure difference between the two pleural cavities; consequently, collapse of one lung may be tolerated. In others, for example, the dog, cat, and horse, it is very delicate and ruptures readily. Indeed the horse is among those species in which the mediastinum of the dead specimen always presents numerous small openings that place the right and left pleural cavities in communication.

THE LUNGS

The right and left lungs (pulmones,* pl.) are each invaginated into the corresponding pleural sac and are free, except at the roots where they are attached to the mediastinum. They have no fixed size or shape since they comply with respiratory changes in the dimensions of the thorax. The lungs are normally kept expanded by the air pressure within the respiratory tree, and being elastic, they recoil and collapse as soon as air is admitted into the pleural cavities by trauma, surgery, or dissection. They have a soft, spongy texture, and the residual air they contain, even when collapsed, causes them to crepitate when squeezed and to float when placed in water. In contrast, the unexpanded lungs of the fetus or stillborn animal feel solid; they sink when immersed, and this provides the pathologist with an easy means of determining that the animal from which they came had not breathed. The color of healthy lungs varies in intensity with the blood content and therefore with the manner of death; it is a fresh pink in many slaughterhouse specimens but a much deeper red in lungs obtained from animals that were not bled. The frequently patchy coloration is produced by uneven distribution of blood, which is often the result of gravitation after death. The lungs of animals that spent their lives in heavily polluted atmospheres acquire a grayish tinge from deposition of soot or other inhaled particles.

Anatomical descriptions are generally based on specimens hardened in situ before the thorax was opened; at death such lungs retain their size, which is intermediate between those adopted in full inspiration and full expiration (Figure 4–22). The two lungs are grossly alike and mirror each other in shape, although the right one is always larger; this asymmetry, partly due to the skewed position of the heart, is most obvious in the lungs of cattle. Each has some resemblance to the half of a cone, which makes it possible to recognize the following features: an apex presented toward the thoracic inlet; a wide, concave base related to the face of the diaphragm; a convex costal surface fitted against the lateral chest wall; an irregular medial surface modeled on the contents of the mediastinum; a thick dorsal border occupying the gutter between the vertebrae and ribs; and a thin border that comprises a ventral part bordering the costomediastinal recess and a basal (caudoventral) part bordering the costodiaphragmatic recess (Figures 4–20 and 4–22). The ventral part is indented over the heart (cardiac notch; incisura cardiaca).

Certain features of the mediastinal surface and base require further attention. The many indentations carried by the mediastinal surface include the large and deep cardiac impression, which is created by the heart and naturally larger on the left lung because the heart itself is biased to this side. The impression extends to the ventral border, which is deeply notched at this level in most species and which, in turn, allows the heart (or more accurately, the pericardium) direct contact with the thoracic wall (see Figure 4–22). The root of the lung, situated dorsal to the cardiac impression, is formed by the bunching together of the chief bronchus and the pulmonary artery, veins, lymphatics, and nerves within a covering of pleura provided by the reflection of the mediastinal pleura onto the lung. The reflection extends caudal to the root in a tapering fashion that leaves bare an area of lung that is directly joined by mediastinal connective tissue to the corresponding part of its partner. In some species, including the dog and the cat, the empty part of the reflection, which is known as the pulmonary ligament, extends onto the base of the lung, which thus finds additional attachment to the diaphragm. In ruminants and pigs the bronchus that arises from the trachea before its bifurcation together with the associated vessels creates a smaller second root of the right lung (Figure 4–23 and Figure 4–17, B).

The base of the right lung reveals the small accessory lobe, which is separated from the medial surface of the caudal lobe by a fissure that widens at its dorsal limit to accommodate the caudal vena cava in its passage between the caval foramen of the diaphragm and the right atrium. The accessory lobe sits, as it were, astride the vein.

In most species one or more fissures extend into the substance toward the root, dividing each lung into parts that are commonly equated with lobes. The lobes are properly defined by the ramification of the bronchial tree, and scope for confusion exists because many older texts employed the external demarcations for this purpose. According to the current practice, the left lung consists of cranial and caudal lobes and the right one of cranial, middle, caudal, and accessory lobes; however, the cranial lobe is commonly subdivided by an external fissure, whereas the right lung of the horse lacks a middle lobe. The fissures are much deeper in the lungs of the dog and the cat than in those of other species, but it is difficult to find convincing functional significance in such differences. The deeper fissures may allow the parts to slip over each other more easily and facilitate the adaptation of the lungs to the pronounced changes in thoracic form that occur in animals that employ a bounding gallop.

The bulk of the lung substance is provided by the bronchi, pulmonary vessels, and peribronchial and perivascular connective tissue. The right and left chief bronchi arise at the tracheal bifurcation above the heart, and after entering the lung at its root, each detaches a bronchus to the cranial lobe before continuing caudally (see Figures 4–17 and 4–24). The two generations of subdivisions that follow next have a fairly consistent pattern of origin, but subsequent ramifications are less predictable. The number of bronchial generations before the smaller bronchi are succeeded by bronchioles varies among species and also among parts of the one lung. In mice and other small animals only four or five generations of bronchi are present, whereas more than a dozen may be necessary in large animals. The consistency in the pattern of the first branchings allows the recognition of the so-called bronchopulmonary segments, specific portions of the lung supplied by identifiable bronchi and partly defined by connective tissue septa that extend from the peribronchial and perivascular tissue (and are responsible for the surface marbling where they impinge on the visceral pleura). Although bronchopulmonary segmentation has been studied in domestic species, it has yet to find important application; it is not yet common veterinary practice to resect portions of diseased lungs. It is the elasticity of the connective tissue stroma that allows the lungs to expand on inspiration and collapse on subsequent expiration. Loss of this elasticity, which occurs naturally with aging (but also in certain pathological conditions), reduces respiratory efficiency.

The structure of the major bronchi resembles that of the trachea, but with each successive division the supporting cartilages become smaller and more irregular, while the muscle expands to enclose the lumen on all sides. The lumen is lined by a pseudostratified epithelium comprising tall ciliated columnar cells interspersed with goblet and serous-secreting cells and with stem cells that proliferate to repair depletions of the other types. Larger glands are included within the submucosa of the major bronchi. The transition from bronchus to bronchiole is defined by the disappearance of the last cartilage plate and by the submucosal glands. Bronchioles are narrow—less than 1 mm in diameter—and also pass through several generations. The last of these is characterized by the loss of goblet cells and their replacement by the exocrinocytes (Clara cells) thought to secrete a component of lung surfactant. The terminal bronchioles present scattered alveolar outpouchings of their walls (and are continued by alveolar ducts), thence alveolar sacs, and ultimately by the saclike alveoli, the spaces where gaseous exchange takes place through a flattened epithelium closely related to the pulmonary capillaries. Patency of the finer passages, which are unsupported by cartilage, is ensured by elastic fibers that anchor them to the pulmonary stroma. At the first breath, the alveoli fill with air and dilate, although for a time they remain significantly smaller than those of the adult (Figure 4–25).

The identification of the lungs of individual species is most conveniently based on the degrees of lobation and lobulation. The lungs of horses show almost no lobation and very inconspicuous lobulation externally (Figure 4–26), those of ruminants (Figure 4–27) and pigs are conspicuously lobated and lobulated (though not uniformly in sheep and goats), and those of carnivores are very deeply fissured into lobes but show little external evidence of lobulation (see Figure 4–23).

The pulmonary arteries generally follow the bronchi (see Figure 4–24), while the pulmonary veins sometimes run separately, alternating in position with the bronchoarterial associations. The pattern varies not only with the species but also with location in the one lung. These differences may find clinical significance if lung surgery becomes more common. Then it will be important to know the vascular arrangements and to be aware that both interarterial and intervenous anastomoses are to be found crossing the connective tissue partitions. A set of bronchial arteries arises from the aorta to supply the bronchi and associated connective tissue wholly independently of the pulmonary arteries (Figure 4–28). A corresponding set of bronchial veins may return this blood to the right atrium via the azygous vein, but often the bronchial flow is entirely returned to the left atrium. Arteriovenous anastomoses appear to be absent, and this makes the lung an effective filter for preventing the further spread of emboli and tumor cells. This accounts for the frequent occurrence of abscesses and tumor metastases in lung tissue, secondary to disease of other organs.

Lymph drains to the tracheobronchial and mediastinal lymph nodes, directly or after initial passage through small pulmonary nodes set on the bronchial tree within the lung substance; the details are complicated, vary among species, and receive later notice when of pathological relevance.

The nerves to the lungs are delivered through a pulmonary plexus within the mediastinum to which both sympathetic and parasympathetic (vagal) fibers contribute. The efferent fibers pass to the bronchial glands and musculature and to the blood vessels. Afferent fibers come from the bronchial mucosa (cough reflex), from vessels, and from stretch receptors. Vagal section has been found to relieve pain in inoperable bronchial carcinoma of human patients.

The features of the lungs of greatest clinical significance are their projection on the surface of the body and their radiographic appearance. The projections vary among species and are described later; meanwhile, it may be stressed that they obviously vary with the phase of respiration. Moreover, the areas over which auscultation and percussion can usefully be employed are more limited than might initially be supposed; this is partly because intervention of the upper part of the forelimb denies access to part of the lung field and partly because the lower border of the lung is too thin to provide much useful information.

Because radiography of the lungs is done mainly in small animals (dogs and cats), the relevant observations on their appearance on radiographs and figures will be found in Chapter 13.

THE DEVELOPMENT OF THE RESPIRATORY APPARATUS

The development of the nose was considered in the previous chapter in relation to the development of the mouth and face (p. 141). The larynx, trachea, and lungs find a common origin in a ventral outgrowth from the foregut, directly caudal to the second of the two swellings that form the tongue (Figure 4–29). The primordium extends caudally as a (tracheobronchial) groove in the pharyngoesophageal floor; the groove is later converted into a tube by infolding and fusion of its lips. Fusion commences caudally and extends forward until the esophagus and pharynx are divided from the respiratory tract, except for a small cranial opening that persists as the entrance to the larynx. The fact that the initial development has the form of a groove rather than a tube is important because it explains the wide variety of communications between the esophagus and trachea that may occur as congenital anomalies when the process of division has been locally unsuccessful.

The further differentiation of the larynx includes the appearance of the separate cartilages and muscles by condensation and differentiation of the mesoderm of the neighboring pharyngeal arches. The epiglottis has a somewhat different origin, developing as a caudal division of the second of the two median swellings that give rise to the tongue.

After separation from the esophagus, the caudal end of the respiratory tract grows down the neck and comes to lie in the median mesoderm that intervenes between the two forward-pointing extensions of the celom that become the pleural cavities. The apex of the tract splits into two lung buds (Figure 4–29, B), whose further splitting first reproduces the pattern of the bronchial tree and then creates the smaller respiratory passages that succeed the bronchi. In babies about 18 divisions succeed the stem bronchi by the time of birth; however, the process is not yet complete, and further divisions are added during infancy. The branches of the lung buds become invested by the splanchnic mesoderm into which they thrust, and it is this mesoderm that forms the tissues of the respiratory organs other than the lining epithelium (which is, of course, supplied by the foregut endoderm). The histological development of the lungs encompasses three phases named after the dominant microscopic characters: the first (glandular) phase establishes the bronchial pattern, the second (canalicular) phase establishes the respiratory portion of the lung, and the third and final (alveolar) phase is concerned with the development of the alveoli.

The production of surfactant, a substance secreted by certain alveolar cells that reduces the surface tension to allow alveolar expansion when breathing commences, is of rather late occurrence. The respiratory distress syndrome of the newborn is associated with immaturity of this feature of development.