Part 147

Organ hardened in situ. ]

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; 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.

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.

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. 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.

FIG. 371.—RIGHT LUNG OF PIG, MEDIASTINAL ASPECT.