MAMMALIA. 291 



front of the heart, divides into right and left branches for the 

 corresponding lungs. 



Veins. Caval, hepatic portal, and pulmonary systems can be 

 distinguished. 



(a) Caval System. Right and left precavals (anterior venae 

 cavse) and a postcaval (posterior vena cava) carry the impure 

 blood from the general system into the right auricle. The 

 precavals drain the head, neck, and fore-limbs, the postcaval the 

 rest of the body. 



Each precaval is formed by the union of an external jugular, with a 

 subdaviaii vein. The former runs back along the side of the neck from the 

 angle of the jaw, where it is formed by the union of facial veins returning 

 blood from the outside of the head. Just before uniting with the sub- 

 clavian it receives the small internal jugular vein from the brain. The 

 subclavian vein brings back blood from the fore-limb. The right precaval 

 is joined by an unpaired azygos vein, which lies just beneath the vertebral 

 column in the thorax, and carries off blood from a large part of its walls. 

 The left precaval receives a coronary vein from the heart-walls, just before 

 entering the right auricle. 



The postcaval is a very large vein formed at the posterior end of the 

 abdomen by the union of the internal iliac veins from the backs of the 

 thighs. It is joined almost at once by the external iliac veins, which return 

 blood from the hind-limbs and bladder (also uterus in the female). They 

 are direct continuations of the femoral veins of the thighs. The postcaval 

 runs forwards in the abdomen close to and on the right of the aorta, 

 receiving successively ilio-lumbar, spermatic (or ovarian), and renal veins, 

 from the body-walls, spermaries (or ovaries), and kidneys respectively. 

 Before reaching the diaphragm it turns ventralwards, runs through the 

 dorsal part of the liver (from which it receives four chief hepatic veins), and 

 pierces the diaphragm ventral to the gullet. In the thorax it runs forwards 

 between the lungs and finally enters the right auricle. 



(b) Hepatic Portal System. The portal vein is formed by the 

 union of lieno-gaxtric, duodenal, anterior mesenteric, and posterior 

 mesenteric veins, which return blood from the stomach and spleen 

 duodenum and pancreas ileuin and most of large intestines 

 and last part of rectum, respectively. The portal vein soon 

 divides into branches, which break up in the various liver-lobes. 



(c) Pulmonary System. There are two pulmonary veins from 

 each lung. All four open into the dorsal side of the left auricle. 



The muscle of the heart (Fig. 87) is composed of transversely- 

 striated fibres, devoid of sarcolemma, which are united into a 

 close network. This is marked into short lengths corresponding 

 to the constituent cells, and in each of these a muscle-corpuscle, 

 not placed superficially, is imbedded. The arteries and veins are 



