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 Monotrenies (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 

 the 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. C. 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 by a transverse arched partition, the diaphragm (D), 

 composed of a thicker peripheral muscular portion and a 

 thinner central tendinous part. Through the latter pass 

 the great blood-vessels and the oesophagus. The anterior 

 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, passing 

 up through the diaphragm and uniting opposite the heart 

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

 emptying into the right auricle. The oesophagus (Oe) 

 overlies the trachea (Tr). The aorta arises from the heart, 

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

 both trachea and oesophagus, as indicated by the dotted 

 lines, and continues its backward course just below the vena 

 azygos, into the abdomen. The trachea gives off a bronchus 

 to each lung (Lu). The lungs are sacculated elastic organs, 

 with no main central cavity. They are separated dorsally by 

 a thin median vertical membrane (M), the mediastinum, the 

 equivalent of the mesentery in the abdomen. Lying on the 

 side of the vertebral column can be seen part of one of the 

 two chains of sympathetic nervous ganglia (S). 



