674 MAMMALIA. 



an anterior intercostal from the spaces between the anterior 



ribs, 



an internal mammary from the ventral wall of the thorax ; 

 and the right superior vena cava also receives an azygos cardinal vein, 

 which runs along the mid-dorsal line and collects blood from the 

 posterior intercostal spaces. 



The inferior vena cava is a large median vein lying beside the aorta 

 beneath the backbone. Anteriorly it is embedded in the liver, and re- 

 ceives the hepatic veins. Thence it passes through the diaphragm into 

 the right auricle. Posteriorly the inferior vena cava has the following 

 components : 



internal iliacs from the back of the thighs, forming by their union 



the beginning of the inferior vena cava ; 

 femoral veins from the inner borders of the thighs, continued into 



external iliacs which open into the inferior vena cava ; 

 paired ilio-lumbars from the posterior abdominal walls ; 

 spermatic or ovarian veins from the reproductive organs ; 

 renal veins from the kidneys. 

 There is no renal portal system. 



The food which has been digested rendered soluble and diffusible 

 passes from the food canal into the vascular system by two paths : 



(a) All except the fatty material is absorbed by veins from the stomach 



and intestine. These unite in a main trunk the portal vein. The 

 components of the portal vein are the lieno-gastric from the 

 stomach (and also from the spleen), the duodenal from the duo- 

 denum (and also from the pancreas), the anterior mesenteric from 

 the intestine, the posterior mesenteric from the rectum. The 

 portal vein breaks up into branches in the liver, whence the 

 modified blood passes by hepatic veins into the inferior vena 

 cava. 



(b] The fat passes through the intestinal villi into the lymphatic 



vessels, which combine to form a thoracic duct which runs for- 

 ward, and opens into the left subclavian vein at its junction with 

 the left external jugular. Here and there lie lymphatic glands. 



Respiratory System. 



The lungs are pink, spongy bodies, lying in the thorax, 

 connected to the exterior by the bronchial tubes and the 

 trachea, and to the heart by blood vessels. The pleural 

 membrane which invests the surface of the lungs is reflected 

 from the sides of the thoracic cavity. When the lungs 

 expand, the pleural cavity between the two folds of pleural 

 membrane is almost obliterated. The thoracic cavity is 

 separated from the abdominal cavity by a partly muscular 

 diaphragm, which is supplied by two phrenic nerves, arising 

 from the fourth cervical spinal nerves. By its contraction 

 the diaphragm alters the size of the thoracic cavity, and 



