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 toAvn. 



Accessory fissures may partially subdivide some of the lobes; this is most common in re- 

 gard to the apical lobe. When the kmgs 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 ^'ena 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 oesophagus behind the root; (5) a groove for the internal thoracic vessels cur\-- 

 ing downward and backward over the apex; (6) a groove for the right plu-enic nerve in the upper 

 part of the cardiac impression; (7) grooves for the dorso-cervical and vertebral veins 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 internal thoracic \'es- 

 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 rmis 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 cm.) 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 cm.) 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 foiTu; they are situated on the lateral surfaces of the trachea near 

 the larjmx, extending along the first six or seven rings (Fig. 4.34). 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 cm.) 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 thj^roid 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. 



