654 MAMMALIA. 



The two superior venae cavse bring blood from the head, neck, thorax, 

 and fore-limbs. Each is formed from the union of — 

 a subclavian from the shoulder and fore-limb, 

 an external jugular from the face and ear, 

 an internal jugular from the brain, 



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 

 receives 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 

 duodenum (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. 

 (/>) The fat passes through the intestinal villi into the lymphatic 

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

 forward 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 with the exterior by the 

 bronchial tubes and the trachea, and with 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 



