Part 148

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.

The length of the =nasal cavity= varies greatly in different breeds, corresponding, of course, to the length of the face. The cavity is roomy, but is very largely occupied by the turbinals and the lateral masses of the ethmoid bone. The =middle meatus= is short and narrow, and divides posteriorly into two branches; the upper branch leads to the ethmoidal meatuses; the lower branch joins the inferior meatus. The =inferior meatus= is very small in its middle part, owing to the great development here of the inferior turbinal. The posterior part of the nasal cavity is divided by a horizontal plate, the lamina transversalis, into an upper olfactory part and a lower respiratory part. The sinuses have been described (vide Osteology).

FIG. 372.—NASAL CARTILAGES OF DOG.

a, Upper parietal cartilage; b, lower parietal cartilage; c, alar cartilage. (After Ellenberger, in Leisering’s Atlas.) ]

THE LARYNX

The larynx is relatively short. The lamina of the =cricoid cartilage= is wide; the arch is grooved laterally. The laminæ of the =thyroid cartilage= are high, but short; they unite ventrally to form the body, on which there is anteriorly a marked prominence, and posteriorly a deep notch. The oblique line on the outer surface of the lamina is prominent. There is a rounded notch (Fissura thyreoidea) below the short anterior cornua for the passage of the superior laryngeal nerve. The posterior cornu is strong, and has a rounded surface for articulation with the cricoid cartilage. The =arytenoid cartilages= are relatively small, and have between them a small =interarytenoid cartilage=. The =epiglottis= is quadrilateral; its lower part or stalk is narrow, fitting into the angle of the thyroid cartilage. The =cuneiform cartilages= are large and somewhat crescent-shaped; they are not blended with the epiglottis.

FIG. 373.—SAGITTAL SECTION OF NASAL REGION OF DOG.

1, Skin; 2, parietal cartilage; 3, floor of nasal cavity; 4, lamina transversalis; 5, mucous membrane of hard palate; 6, upper lip; 7, frontal bone; 8, frontal sinus; 9, cranial plate of frontal bone; 10, cranial cavity; 11, dotted line indicating anterior limit of olfactory region; a, superior turbinal; b, inferior turbinal; b′, b″, inferior turbinal fold; c, e, ethmoturbinals; d, cribriform plate of ethmoid; f, superior meatus; g, middle meatus; h, inferior meatus; i, naso-pharyngeal meatus. (After Ellenberger, in Leisering’s Atlas.) ]

The =false vocal cords= extend from the cuneiform cartilages to the thyroid. The =true vocal cords= are large and prominent. The entrance to the large lateral ventricle and saccule is a long slit parallel with the anterior margin of the true vocal cord.

The crico-thyroid muscle is thick. The hyo-epiglotticus is well developed, and is double at its hyoid attachment. The anterior part of the thyro-arytenoideus arises on the cuneiform cartilage. Hence Lesbre has suggested the name cuneo-arytenoideus for it.

THE TRACHEA AND BRONCHI

The trachea is very slightly flattened dorsally. It contains forty to forty-five =U=-shaped rings; the ends of the rings do not meet dorsally, so that here the wall of the trachea is membranous and is composed of a layer of transverse smooth muscle-fibers outside of the rings, the fibrous membrane, and the mucous membrane.

The stem bronchi diverge at an obtuse angle, and each divides into two branches before entering the lung. The anterior branch goes to the apical and cardiac lobes, the posterior one to the diaphragmatic lobe, supplying also the mediastinal lobe of the right lung. The anterior branch of the left bronchus crosses under the pulmonary artery.

THE LUNGS

The lungs (Figs. 342, 343) differ in shape from those of the horse and ox in conformity with the shape of the thorax, which is relatively very wide in the dog; the lateral thoracic walls are strongly curved, and the costal surface of the lungs is correspondingly convex. There is no cardiac notch on the left lung, although a small part of the pericardium is not covered by the lung at the lowest part of the fifth and sixth intercostal spaces. On the right side the cardiac notch is opposite the fourth and fifth intercostal spaces. Each lung is divided by deep fissures into =three lobes=—=apical=, =cardiac=, and =diaphragmatic=; the right lung has in addition a =mediastinal lobe=. The apical lobe of the right lung is considerably larger than that of the left. The latter is often more or less fused with the cardiac lobe. Variable accessory lobes may be found. On account of the small amount of interlobular tissue the lobulation is not distinct.

THE THYROID GLAND

The =lateral lobes= of the thyroid gland are long and narrow, and have a flattened, ellipsoidal form; they are placed on the lateral surfaces of the trachea near the larynx. The extremities are small, the posterior one often being pointed. The =isthmus= is inconstant and variable; in large dogs it usually has the form of a glandular band which may be nearly half an inch (ca. 1 cm.) wide; in small dogs it is usually absent.

=Accessory thyroids= are frequently present; three or four may be found on either side, as well as a median one near the hyoid bone.

The =parathyroids=, about the size of millet or hemp seed, are four in number usually. Two are placed on the deep face of the thyroid lobes; the others are placed laterally near the anterior extremity of the thyroid.

THE THYMUS

The thymus is relatively small, and is situated almost entirely in the thorax. The left lobe is much larger than the right, and extends back as far as the sixth rib at the time of its greatest size. According to Baum the gland increases in size during the first two weeks after birth, and atrophies rapidly in the next two or three months. Traces of it are present at two or three years of age, and may be found even in old subjects.