564 Z00LO0GY. 
(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 (O¢aria). 
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. ©. 8. 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 (J), 
composed of a thicker peripheral muscular portion and a 
thinner central tendinous part. Through the latter pass 
the great blood-vessels and the cesophagus. 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 (H?¢) was en- 
closed in the thin-walled pericardial sac, which has been 
cut away. The great systemic veins enter from behind— 
d.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 cesophagus (Oe) 
overlies the trachea (Zr). The aorta arises from the heart, 
and, curving upward and backward, runs to the left of 
both trachea and csophagus, 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 (Zu). The lungs are sacculated elastic organs, 
with no main central cavity. They are separated dorsally by 
a thin median vertical membrane (J), 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 (8). 
