226 VERTEBRATE ZOOLOGY 



the internal mammary artery, a branch of the subclavian artery 

 in the forward part of the body. The continuation of the external 

 iliac is called the femoral artery ; this artery, with its branches, 

 supplies the greater part of the leg. Follow these branches. 



The internal iliac, or hypogastric, arteries are paired vessels 

 which leave the aorta a short distance behind the external iliacs ; 

 each sends branches to the bladder, rectum, urogenital organs, 

 and muscles. 



The caudal, or sacralis media, artery is the small continuation of 

 the aorta which supplies the tail. 



Exercise 16. Draw a diagram showing the abdominal aorta and its 

 branches so far as observed. 



The Thoracic Cavity. Cut away the ventral and lateral walls 

 of the thorax and thoroughly expose its cavity, but do not remove 

 the diaphragm. The heart will be seen in its pericardium. Im- 

 mediately in front of the pericardium is the thymus gland, an 

 elongated pinkish organ of uncertain function. It varies much 

 in size in different animals ; in young animals it may project over 

 the pericardium, while in old animals it is much smaller and may 

 be almost entirely wanting. On each side of the heart will be 

 seen the pleural sac with the lung. 



Examine the diaphragm carefully. Note its position in the 

 body cavity. Its general form is cup-shaped, the opening of the 

 cup looking toward the abdominal cavity. It will be seen to 

 consist of a thin plate of muscle surrounding a transparent central 

 tendon, the semilunar tendon. The muscular portion is made up 

 of two parts,— the dorsal part, and the ventral and lateral part. 

 The former, which is called the vertebral portion, extends from the 

 semilunar tendon back to the vertebral column, the general direc- 

 tion of the fibers being longitudinal. The latter, the sternocostal 

 portion, forms the larger part of the diaphragm ; its fibers radiate 

 from the semilunar tendon to the lateral and ventral sides of the 

 thorax. The postcaval vein passes through the semilunar tendon ; 

 the oesophagus and aorta, through the vertebral portion. 



Remove the diaphragm. Make a hole in the pericardium and 

 blow into it with the blowpipe, to determine its extent. Remove 



