Part 146

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.

=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.

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.

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.

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.

FIG. 367.—RIGHT LUNG OF OX, COSTAL SURFACE.