Part 49
The arrangement of the larger bronchi is not the same in the two lungs. The right bronchus, after detaching the apical branch, gives off a large ventral branch which shortly bifurcates. One of the divisions passes ventrally into the part of the lung behind the cardiac notch (homologue of cardiac lobe), while the other passes downward and backward into the lower part of the base. The stem bronchus then gives off the branch to the mediastinal lobe, and divides, about six inches from the hilus of the lung, into two nearly equal branches; the dorsal branch is a direct continuation of the stem, the ventral one passes into the middle part of the base. The left bronchus, after a short course, divides into two nearly equal branches. The dorsal branch may be regarded as the continuation of the stem; it passes backward parallel with the dorsal border of the lung, giving off a ventral branch much further back than in the right lung. The ventral division of the stem bronchus divides, after a very short course, into two branches; the anterior one passes ventrally into the homologue of the cardiac lobe, while the posterior one passes downward and backward into the base of the lung.
The =structure= of the larger bronchial tubes is, in general, similar to that of the trachea. Their walls contain irregular plates of cartilage instead of rings. There is a continuous layer of unstriped muscle, composed of circularly arranged bundles. The mucous membrane presents numerous longitudinal folds; it contains many elastic fibers, mucous glands, and lymph nodules, and is lined by ciliated columnar epithelium. As the tubes diminish in size the coats become thinner and the cartilages smaller; in tubes about 1 mm. in diameter the cartilages and mucous glands are absent.
By repeated branching the =interlobular bronchi= are formed, and from these arise the =lobular bronchioles=. The latter enter a lobule and branch within it, forming the =respiratory bronchioles=; these give off the =alveolar ducts=, the walls of which are pouched out to form hemispherical diverticula, the =alveoli= or =air-cells=.
A =lobule= of the lung, the unit of lung structure, is made up of a lobular bronchiole with its branches, and their air-cells, blood- and lymph vessels, and nerves. Between the lobules is the =interlobular tissue=, which forms the supporting framework of the lung.
=Vessels of the Lungs.=—The branches of the =pulmonary artery= carry venous blood to the lungs. They accompany the bronchi, and form rich capillary plexuses on the walls of the alveoli. Here the blood is arterialized, and is returned to the heart by the =pulmonary veins=. The =bronchial arteries= are relatively small vessels which carry arterial blood for the nutrition of the lungs. The branches of these arteries accompany the bronchial ramifications as far as the alveolar ducts, but do not extend to the alveoli. The œsophageal artery also supplies pulmonary branches (in the horse) which reach the lung by way of the ligament of the latter. These branches vary much in size, and ramify chiefly in the subpleural tissue of the basal part of the lung.[131] The =lymph vessels= are numerous, and are arranged in two sets. The superficial set forms close networks in and under the pleura, while the deep set accompanies the bronchi and pulmonary vessels. All converge to the root of the lung and enter the =bronchial lymph glands=.
=Nerve-supply.=—The pulmonary nerves come from the vagus and sympathetic nerves. They enter at the hilus and supply branches to the bronchial arteries and the air-tubes.
THE THYROID GLAND
The =thyroid gland= (Glandula thyreoidea) is a very vascular ductless gland, situated on the trachea close to the larynx. It is red brown in color, and consists of two =lateral lobes= and a connecting =isthmus= (Fig. 354).
The =lateral lobes= are situated on either side of the first and second or second and third rings of the trachea, to which they are loosely attached. Each is about the size of a walnut, and has a convex superficial face which is related to the parotid gland and the omo-hyoid muscle, and a slightly concave, deep face, applied to the trachea.
The =isthmus= in the adult horse is usually very rudimentary. It may occur as a very narrow glandular band which connects the posterior extremities of the lateral lobes, extending across the ventral surface of the trachea, but it is frequently only a small strand of connective tissue, and sometimes is entirely absent. It may be represented by a small tail-like process of one lobe.
The lateral lobes vary in size and position, and are often dissimilar on the two sides. The anterior extremity is the larger; it is rounded and usually lies about a finger’s breadth behind the larynx, but may be in contact with it. The posterior extremity is often produced to form a narrow tail-like process, which is continuous with the isthmus when one is present. In the ass and mule the isthmus is usually well developed. There is sometimes a narrow band of thyroid tissue which extends forward from the isthmus for a variable distance; this is termed the pyramidal lobe. The position of the lateral lobe is indicated approximately by the junction of the external maxillary and jugular veins.
=Structure.=—The gland is enveloped by a thin, fibrous =capsule=, from which =trabeculæ= pass into the gland, dividing it into =lobules=. The lobules consist of non-communicating =alveoli= or =follicles= of varying form and size. The alveoli are lined by cubical epithelium, and contain a viscid colloid substance.
=Blood-supply.=—The arteries are relatively very large; there are usually two thyroid arteries, which arise from the carotid and enter each extremity of the gland.
=Nerve-supply.=—The nerves are derived from the sympathetic system.
Nodules of thyroid tissue of variable size, sometimes as large as a pea, may be found near the anterior extremity of the lateral lobes, on the course of the anterior thyroid artery or on the trachea, even at a considerable distance from the thyroid region. They are termed =accessory thyroids= (Glandulæ thyreoidæ accessoriæ).
The =parathyroids= are small glandular bodies found in the thyroid region. They are paler and not so dense as the thyroid tissue, but often cannot be differentiated by their naked-eye appearance. In the horse there is usually only one on each side in the connective tissue over the anterior extremity of the lateral lobe of the thyroid. They resemble in structure embryonic thyroid tissue.
THE THYMUS GLAND
The =thymus= is a ductless gland which has a close resemblance to the lymphoid tissues. It is well developed only in late fœtal life and for a few months after birth. After this it undergoes rapid atrophy, fatty infiltration, and amyloid degeneration, so that in the adult it is represented by a thin remnant in the lower part of the anterior mediastinum or has entirely disappeared. In the new-born foal it is of a grayish-pink color, and consists of two =lateral lobes= (Fig. 358). The greater part of the gland is situated in the anterior mediastinum, but the two lobes are continued into the neck by a chain of lobules which lie along the course of the carotid artery, extending sometimes as far as the thyroid gland.[132]
=Structure.=—The thymus is inclosed in a thin, connective-tissue =capsule=, and consists of secondary =lobules= (Lobuli thymi), of varying shape and size, held together by areolar tissue and fat, and all attached to a cord of adenoid tissue termed the =tractus centralis=. These lobules are subdivided into polyhedral primary lobules, which are composed of lymphoid tissue, and present a dark vascular =cortex=, and a lighter =medulla= that contains Hassal’s corpuscles.
=Blood-supply.=—Internal thoracic and carotid arteries.
=Nerve-supply.=—Sympathetic and vagus nerve.
RESPIRATORY SYSTEM OF THE OX
THE NASAL CAVITY
The =nostrils=, situated on either side of the muzzle, are relatively small, and are much less dilatable than those of the horse. The alæ are thick and firm. The superior commissure is narrow. There is no false nostril or diverticulum nasi. The skin is smooth, bare, and moist, and there is no clear line of demarcation between it and the nasal mucosa. The opening of the naso-lacrimal duct is not visible, as it is on the outer wall of the nostril, and is concealed by the cartilaginous prolongation of the inferior turbinal. There are two =parietal= or =lateral cartilages= on either side, united by fibrous tissue. The dorsal cartilages are thin laminæ which curve outward and downward from the dorsal margin of the septal cartilage; they are prolongations of the nasal bones, and carry the alar cartilages on their anterior extremities. The ventral pair are lateral continuations of the basal lamellæ of the inferior turbinals. They lie along the nasal processes of the premaxillæ; anteriorly, they become thicker, turn a little upward, and each blends with the cartilaginous prolongation of the upper coil of the inferior turbinal. The =alar cartilages= are of peculiar form. The lamina is oblong, and curves outward and downward from the anterior extremity of the dorsal parietal cartilage, with which it is connected. The cornu springs from the outer part of the lamina and curves outward and upward into the outer wing of the nostril; it carries on its extremity a small transverse bar, thus having some resemblance to the fluke of an anchor.
The =nasal cavity= is short, wide anteriorly, narrow behind. It is not completely divided by the septum, which in its posterior third is separated from the floor of the cavity by an interval that increases from before backward. The middle meatus is very narrow, and divides posteriorly into two branches; the upper division leads to the ethmoidal meatuses, and communicates with the frontal sinus and the cavity of the superior turbinal. Anterior to the division are communications with the upper cavity of the ventral turbinal and the maxillary sinus. The main facts in regard to the sinuses were stated in the osteology.
[Illustration:
FIG. 364.—MUZZLE OF OX.
_49_, Naso-labial region; _45_, lower lip. (After Ellenberger-Baum, Anat. für Künstler.) ]
THE LARYNX
The =larynx= is more compact than in the horse, and presents numerous differential features. The =cricoid cartilage= is compressed laterally. The lamina is not distinctly marked off from the arch; it slopes downward and backward and has a large median ridge. The =thyroid cartilage= is complete ventrally, _i. e._, the laminæ are united throughout to form a long plate which is notched in front and behind; the laryngeal prominence is small and is situated posteriorly. The height and thickness of the cartilage increase from before backward. The posterior cornua are about an inch long, and form syndesmoses with the cricoid cartilage; the short anterior cornua unite similarly with the hyoid bone. The =epiglottis= is oval in outline and its apex is rounded; its base rests on the thyro-hyoid membrane, to which it is rather loosely attached. The cuneiform cartilages are absent. The =arytenoid cartilages= present only slight differences. The muscular process is well developed, and the vocal process is narrow and long. The =vocal cords= project very little from the wall, so that the rima glottidis is wide; the vocal ligament is attached ventrally to the crico-thyroid membrane; it has the form of a half tube, open behind. The =lateral ventricles= are extremely shallow, and the saccules are absent. The thyro-arytenoideus muscle is not divided; it is fan-shaped, thick, and narrow dorsally; ventrally, it is attached to the base of the epiglottis, the angle of the union of the thyroid laminæ, and the crico-thyroid membrane. The hyo-epiglotticus muscle is large and bifid.
THE TRACHEA
The caliber of the trachea is relatively small, its width being about an inch and a half (ca. 4 cm.) and its height one and a half to two inches (ca. 4 to 5 cm.). The rings are smaller, and number about fifty. In the neck their free ends are separated by a varying interval, so that the tube is flattened and membranous dorsally. In the thorax the ends are in apposition, so that they form a ridge dorsally. The trachea is adherent to the right lung from the second rib backward. The bifurcation is opposite the fourth intercostal space.
[Illustration:
FIG. 365.—SAGITTAL SECTION OF HEAD OF COW.
_1_, Cerebral hemisphere; _2_, lateral ventricle; _3_, thalamus; _4_, corpora quadrigemina; _5_, optic chiasma; _6_, pituitary body; _7_, pons; _8_, medulla oblongata; _9_, cerebellum; _10_, spinal cord; _11_, ventral straight muscles; _12_, pharyngeal lymph gland; _13_, arytenoid cartilage; _14_, epiglottis; _15_, cricoid cartilage; _16_, vocal cord; _17_, palatine sinus; _18_, soft palate; _19_, Eustachian opening; _C_{1}_, _C_{2}_, _C_{3}_, first, second, and third cervical vertebræ; _B.o._, basioccipital; _Sp._, presphenoid. ]
THE BRONCHI
There are three chief bronchi. The bronchus for the apical and the anterior cardiac lobe of the right lung is detached from the trachea at a considerable distance anterior to the bifurcation.
THE THORACIC CAVITY AND PLEURA
The thoracic cavity is relatively small; it is especially short dorsally, and is diminished laterally by the mode of attachment of the diaphragm to the ribs (_vide_ Myology). The endothoracic fascia is better developed than in the horse, and is distinctly elastic. The pleura is also thick, and there are no perforations of the mediastinum. The pleural sacs are more unequal in extent than in the horse, and the ventral part of the mediastinum is further to the left; in front of the pericardium it lies for the most part on the left wall of the chest.
[Illustration:
FIG. 366.—CROSS-SECTION OF THORAX OF SHEEP, PASSING THROUGH MIDDLE OF FIFTH PAIR OF RIBS, POSTERIOR VIEW.
_1_, _1′_, Lungs; _2_, _2′_, bronchi; _3_, _3′_, pulmonary arteries; _4_, thoracic aorta; _5_, œsophagus; _6_, lymph gland; _7_, vena hemiazygos; _8_, posterior vena cava; _9_, large pulmonary vein; _10_, left atrium, _11_, vena hemiazygos; _12_, left ventricle; _13_, right ventricle; _14_, sternum; _15_, internal thoracic vessels; _16_, sixth thoracic vertebra. ]
THE LUNGS
The difference in size between the two lungs is greater than in the horse, the right lung weighing about half as much again as the left one. The average weight of the lungs is about 7½ pounds (ca. 3 to 4 kg.); they form about ¹⁄₁₇₀ of the body-weight.
[Illustration:
FIG. 367.—RIGHT LUNG OF OX, COSTAL SURFACE.
Organ hardened _in situ_. ]
[Illustration:
FIG. 368.—RIGHT LUNG OF OX, MEDIASTINAL ASPECT.
Hardened _in situ_. _1_, Left bronchus; _2_, pulmonary veins; _3_, pulmonary artery. ]
They are divided into lobes by deep =fissures= (Incisuræ interlobares). The =left lung= is divided into =three lobes=, named from before backward =apical=, =cardiac=, and =diaphragmatic=. The =right lung= has =four= or =five lobes=; the =apical lobe= is much larger than that of the left lung, and occupies the space in front of the pericardium, pushing the mediastinum against the left wall;[133] the =mediastinal lobe= resembles that of the horse, while the =cardiac lobe= is often divided by a fissure into two parts. The apical lobe of the right lung receives a special bronchus from the trachea opposite the third rib. The right lung is adherent to the trachea from the second rib backward.
[Illustration:
FIG. 369.—LUNGS AND HEART OF SHEEP, VENTRAL VIEW.
Specimen hardened _in situ_. Space between heart and lungs was occupied by pericardium and fat. ]
The interlobar fissures begin at the ventral margin of the lung and pass toward the root. Those of the left lung lie opposite to the fourth and sixth ribs. The diaphragmatic lobe is the largest, and has the form of a three-sided pyramid with its base resting on the diaphragm. The cardiac lobe is prismatic and forms the posterior margin of the cardiac notch; its long axis corresponds to the fifth rib. The apical lobe of the left lung is small and pointed; its ventral margin lies on the pulmonary artery and the anterior aorta; below this the space in front of the heart is occupied by the apical lobe of the right lung. The cardiac lobe of the right lung is much larger than that of the left lung, and covers the right face of the pericardium, so that the latter has no contact with the lateral wall of the thorax.
The =lobulation= is very distinct on account of the extremely large amount of interlobular tissue.
In the =sheep= the costal attachment of the diaphragm resembles that of the horse, and the basal part of the lung corresponds thereto. The lobation of the lungs resembles that of the ox, but the lobulation is very indistinct. The pleural sacs extend under the first lumbar vertebra.
THE THYROID GLAND
The lobes of the thyroid gland are more extensive and paler in color than in the horse; they extend further forward, overlapping the crico-pharyngeus muscle to a small extent, and come in contact dorsally with the œsophagus. They are flattened, and have an irregular, lobulated surface. In the young subject the isthmus is well developed, being about half an inch (ca. 1 to 1.5 cm.) in width.
Accessory thyroids may be found, and parathyroids also occur near the posterior extremity or inner surface of the lateral lobes of the thyroid.
[Illustration:
FIG. 370.—RIGHT LUNG OF SHEEP, MEDIASTINAL ASPECT.
Hardened _in situ_. _B_, Bronchus; _P.a._, pulmonary artery; _P.v._, pulmonary vein; _V.c._, posterior vena cava; _L.g._, bronchial lymph gland. ]
THE THYMUS GLAND
The thymus is pale and distinctly lobulated. It is much larger in the calf than in the foal, weighing at five or six weeks about 15 to 25 ounces (ca. 425 to 600 grams). The cervical part is specially large, thick, and compact; it extends to the thyroid gland. It undergoes atrophy slowly and remnants of the thoracic part often remain even in advanced age.
RESPIRATORY SYSTEM OF THE PIG
THE NASAL CAVITY
The =nostrils= are small, and are situated on the flat anterior surface of the =rostrum= or snout (Rostrum suis). The latter is a short cylindrical projection, with which the upper lip is fused, and is circumscribed by a prominent circular margin. The skin on the snout is thin and highly sensitive; it presents small pores, and scattered over it are fine short hairs. In the snout between the nostrils is the =os rostri=, which is to be regarded as a special development of the extremity of the septum nasi in adaptation to the habit of rooting. A plate of cartilage, representing the lamina of the alar cartilage of the horse, curves outward and downward from the upper part of the os rostri and a pointed bar of cartilage curves upward from the lower part of the bone in the outer wing of the nostril. The notch between the nasal bone and the premaxilla is closed in by parietal cartilages which resemble those of the ox.
The =nasal cavity= is long and narrow. It is divided behind by a horizontal plate into an upper olfactory part, which leads to the ethmoidal meatuses, and a lower respiratory part, which is a direct continuation of the inferior meatus. The posterior part of the septum is membranous.
The =turbinal bones= resemble in general those of the ox. The =superior meatus= is exceedingly small. The =middle meatus= is a deep fissure between the two turbinals: it divides posteriorly into two branches; one of these extends upward and backward between the lateral mass of the ethmoid bone and the superior turbinal; the other widens and joins the inferior meatus. The opening between the maxillary sinus and the middle meatus lies in a plane through the last cheek teeth. The middle meatus is continuous with the space inclosed by the dorsal part of the inferior turbinal and with the cavity of the superior turbinal. The dorsal division of the middle meatus presents several openings into the frontal sinus. The =inferior meatus= is relatively roomy; it communicates with the space inclosed by the ventral coil of the inferior turbinal. The opening of the naso-lacrimal duct is found in the posterior part of the inferior meatus. The naso-palatine duct and the vomero-nasal organ (of Jacobson) resemble those of the ox.
THE LARYNX
The larynx is remarkable for its great length and mobility. The cartilages are more loosely attached to each other than in the other animals. The =cricoid cartilage= is thick and compressed laterally; its lamina is long and narrow; its arch is directed obliquely downward and backward. The =thyroid cartilage= is very long; its laminæ are united ventrally and form a median ridge. The anterior cornua being absent, there are no joints formed with the hyoid bone. The posterior cornua are broad, bent inward, and articulate with the cricoid cartilage. The =epiglottis= is relatively very large, and is more closely attached to the hyoid bone than to the rest of the larynx. The middle part of its base is turned forward, and rests on the thyro-hyoid membrane; it is closely connected with the body of the hyoid bone by the hyo-epiglottic ligament and the strong hyo-epiglottic muscle.[134] The apex of the =arytenoid cartilage= is very large, and is divided into two parts at its extremity; the inner part is fused with that of the opposite cartilage. There is a small =interarytenoid cartilage= in the transverse ligament. The =rima glottidis= is very narrow. The =vocal cords= are directed obliquely downward and backward; and each is pierced by a long, slit-like opening, which leads into the large laryngeal saccule. The thyro-arytenoid or vocal ligament is similarly divided into a larger anterior and a smaller posterior part. There is a middle ventricle near the base of the epiglottis. The thyro-arytenoid muscle is very strong and is undivided. The arytenoideus is very small. The crico-thyroid consists of two strata; the superficial layer corresponds to the muscle of the other animals; the deep part consists of transverse fibers.
TRACHEA AND BRONCHI
The trachea is circular in cross-section, and contains thirty or more rings which almost meet dorsally. A special bronchus is detached for the apical lobe of the right lung, as in the ox.
THE LUNGS (Figs. 324, 325)
The =right lung= has =four= or =five lobes=, according to whether the apical is subdivided or not. The =left lung= resembles that of the ox in form and lobation. The lobulation is distinct, but the interlobular septa are thinner than in the ox.
[Illustration:
FIG. 371.—RIGHT LUNG OF PIG, MEDIASTINAL ASPECT.
Hardened _in situ_. _B_, Left bronchus; _V._, pulmonary veins; _A._, pulmonary artery; _l_, lymph glands. ]
THE THYROID GLAND
The two lobes of the thyroid are long and flattened; they are situated close together, on the ventral surface of the trachea, extending as far forward as the cricoid cartilage. The gland is dark red in color.
THE THYMUS
The thymus is very large, extending to the larynx or even to the submaxillary space in young subjects.
RESPIRATORY SYSTEM OF THE DOG
THE NASAL CAVITY
The =nostrils= are situated on the muzzle, with which the upper lip blends. They are shaped somewhat like a comma, with the broad part next to the septum and the narrow part directed backward and outward. The skin around the nostrils is bare, usually black, and in health moist and cool. The muzzle is marked by a median furrow (philtrum) or a deep fissure in some breeds. The cartilaginous framework is formed essentially by the septal cartilage and the parietal cartilages which proceed from it. The septal cartilage projects beyond the premaxilla, and is much thickened at its extremity; it gives off from its upper and lower margins the two parietal cartilages, which curve laterally and toward each other. A grooved plate extends outward from the septal cartilage in the floor of the nostril, and another lamina supports the alar fold of the inferior turbinal bone.