URINARY ORGANS. 49 



trachea and neck. While the Cetacea have only from 7 to 12, the 

 Carnivora have mostly from 30 to 40, and even more rings ; there 

 occur from 60 to 100 in the Rmninantia, and in the Camels even 110 

 rings ; their number varies, however, in individuals of the same 

 species. In Man we find from 17 to 20 imperfect rings. The 

 trachea is almost always straight, t. e., it passes without any con- 

 tortions to the lungs ; in the Sloth (Bradypus tridactylus) alone, it 

 makes, as in many Birds, a bend forward and downward before 

 dividing into the bronchi. The trachei usually divides, as in Man, 

 into two main bronchial trunks ; occasionally, however,, as in the 

 Ruminantia and the Hog, it divides into three, which is also the case 

 in the true Cetacea. The third supernumerary bronchus is always 

 smaller, arises anteriorly, and passes to the right lung. The bron- 

 chial rings are in one case to be traced, quite complete, far into the 

 lung, in others they soon disappear. 



The Lungs are sometimes quite simple and undivided upon each 

 side, as in the Horse, Elephant, and Rhinoceros. The number of 

 the lobes, is, however, commonly greater than in Man, four or even 

 five (as in Hamster and Marmot) being found upon the right, two to 

 three upon the left. The right lung is usually larger, sometimes 

 double the size of the left, as in the Musk-deer. The lungs of all 

 Mammalia, as in Man, have terminal cells which are situated at the 

 extremities of the bronchi, and appear to be in the whole class near- 

 ly of the same size from ^th to -^th of a line. 



The Mammalia, like Man, have a Thymus gland, which is formed 

 toward the end of fcetal existence, and attains its greatest develop- 

 ment during the period of lactation ; it afterward usually disappears 

 by degrees, though sometimes it is persistent. It lies in the antero- 

 superior part of the cavity of the chest, usually consists of two main 

 lobes, and agrees in structure with that of the human thymus. AH 

 diving animals, the Beaver, Seal, Otter, and Cetacea, exhibit a very 

 large thymus gland which exists throughout life, and which frequent- 

 ly extends from the thoracic cavity, along the trachea, up both sides 

 of the neck, as is the case in the Ruminantia. 



URINARY ORGANS. 



IN the internal structure of the Kidneys, in reference to their 

 vascular system, their Malpighian bodies, the infundibuliform struc- 

 ture of the urinary canals, &c., the Mammalia agree with Man. 

 The kidneys in most of the Carnivora are much rounded ; in the 



4 



