THE THYROID GLAND—THE THYMUS 553 
cardiac notch is triangular and allows the pericardium to come in contact with the 
lateral wall at the fourth and fifth interchondral spaces. 
The left lung is divided into three lobes—apical, cardiac, and diaphragmatic. 
The cardiac impression is shallow, and a distinct cardiac notch is not present, but 
the pericardium is in contact with the lateral wall along a narrow area at the ventral 
part of the fifth and sixth interchondral spaces. The apical lobe has a small, blunt- 
pointed apex which lies over the manubrium sterni. On account of the small 
amount of interlobular tissue the lobulation is not distinct. Pigmentation of the 
interlobular tissue is usual in dogs kept in town. 
Accessory fissures may partially subdivide some of the lobes; this is most common in re- 
gard to the apical lobe. When the lungs have been successfully hardened in situ, additional mark- 
ings are seen on the mediastinal surface. On the right lung there are: (1) A groove for the an- 
terior vena cava in front of the root; (2) a groove for the vena azygos which curves upward and 
backward over the root; (3) a shallow groove for the aorta above the posterior part of the root; 
(4) a groove for the cesophagus behind the root; (5) a groove for the internal thoracic vessels curyv- 
ing downward and backward over the apex; (6) a groove for the right phrenic nerve in the upper 
part of the cardiac impression; (7) grooves for the dorso-cervical and vertebral ves running from 
the dorsal border to the groove for the vena cava. On the left lung there are: (1) A well-marked 
groove for the aorta, which curves over the root and runs back just below the dorsal border; (2) 
a shallow groove for the cesophagus behind the root; (3) grooves on the apical lobe for the left 
brachial and brachiocephalic arteries; (4) grooves near the apex for the mternal thoracic ves- 
sels; (5) a groove for the left phrenic nerve is more or less evident; it passes downward and back- 
ward over the middle of the apical lobe, then runs backward over the cardiac and diaphragmatic 
lobes. 
The diaphragmatic line of pleural reflection extends along the lower part of the ninth costal 
cartilage, crosses the tenth cartilage a little (ca. 1-2 em.) below the costo-chondral junction, and 
the eleventh rib at the costo-chondral junction; it then curves up to the last rib, just above its 
middle, and passes medially and backward to the second lumbar transverse process. In a good- 
sized dog (e. g., bull-terrier) the apex of the pleural sac extends about an inch (ca. 2.5 em.) anterior 
to the plane of the first rib. 
THE THYROID GLAND 
The lateral lobes of the thyroid gland are long and narrow, and have a flat- 
tened, ellipsoidal form; they are situated on the lateral surfaces of the trachea near 
the larynx, extending along the first six or seven rings (Fig. 434). The extremities, 
are small, the posterior one often being pointed. The isthmus is inconstant and 
variable; in large dogs it has the form of a glandular band which may be nearly 
half an inch (ca. 1 em.) wide; in medium-sized dogs it is often absent, and 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 situated on the deep face of the thyroid lobes, in which they are 
often embedded; 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 on the pericardium 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. 
