564 ZOOLOGY. 



(Fig. 491, C) is greatly developed in the mammalia, while 

 the external ear now appears. This is a prolongation of 

 the edges of the first branchial cleft of the embryo. There 

 is, however, no external ear in the Monotremes (duckbill). 

 It is also absent in whales, the Sirenians or sea-cows, in 

 most seals, and is very small in the eared seals {Otaria). 

 The eye of mammals is not essentially different from that of 

 tlie lower vertebrates. 



The general anatomy of the soft parts of a mammal may 

 be studied by dissecting a cat, with the aid of the following 

 description and drawings prepared by Dr. 0. S. Minot : 



Fig. 492 illustrates the general anatomy of the cat ; the 

 skin and right half of the body-wall have been removed. 

 The body-cavity is divided into an anterior and posterior 

 division iDy a transverse arched partition, the diaphragm {D), 

 composed of a thicker peripheral muscular portion and a 

 tlrinner central tendinous part. Through the latter pass 

 the great blood-vessels and tlie oesophagus. The anteriol 

 chamber is the thorax or pleural cavity, and contains 

 the respiratory organs and heart. To show these, the 

 right lung has been removed. The heart {Ht) was en- 

 closed in the thin-walled pericardial sac, which has been 

 cut away. The great systemic veins enter from behind — 

 i. e. , dorsally ; from below the vena cava inferior V, passing 

 up through the diaphragm and uniting opposite the heart 

 with the large vein, cava superior, from above, the two 

 emptying into the right auricle. The cesophagus (Oe) 

 overlies the trachea (2>). The aorta arises from tlie heart, 

 and, curving upward and backward, runs to tlie left of 

 both trachea and oesophagus, as indicated by tlie dotted 

 lines, and continues its backward course {Ao) just in 

 front of the vertebral column into the abdomen. The 

 trachea gives off a bronchus to each lung (Lu). The lungs 

 are sacculated elastic organs, with no main central cavity. 

 Tliey are separated dorsally by a thin median vertical mem- 

 brane (If), the mediastinum, the equivalent of the mesen- 

 tery in the abdomen. Lying on the side of the vertebral 

 column can be seen part of one of the two chains of sym- 

 pathetic nervous ganglia (S). 



