THE VEINS. 25 



muscles of the side and back of the body, 

 and of the head as far forwards as the nose. 



"b. The left anterior vena cava corresponds in its course 

 and branches to the right one. 



c. The posterior vena cava is a median vein which, com- 

 mencing between the kidneys, runs forward to open 

 into the posterior end of the sinus venosus. It returns 

 to the heart the blood from the liver and from the 

 kidneys, and indirectly from other viscera and from 

 the hind limbs. It receives the following veins : — 



i. The right and left hepatic veins, from the liver : 

 these open into the jDOsterior vena cava just before 

 it joins the sinus venosus. 



ii. The renal veins, from the kidneys : of these there 

 are four or five on each side which open into, or 

 rather form by their union, the posterior vena 

 cava. The most anterior of these receive the 

 veins from the fat bodies. 



iii. The ovarian veins (in the female), or spermatic 



veins (in the male) ; returning blood from the 

 ovaries or testes. They are usually four or five 

 in number on each side, and open into the 

 posterior vena cava between the renal veins. 



II. Vein opening into the Left Auricle. 



a. The pulmonary vein is formed by the union of the 

 right and left pulmonary veins, returning to the heart 

 the blood from the right and left lungs respectively. 

 Each pulmonary vein runs along the inner side of its 

 lung. 



III. The Portal Systems. 



A portal vein is one which, returning blood from the capil- 

 laries of some part, breaks up before reaching the heart into a 

 second set of capillaries within some other organ ; these again 

 unite to form a vein which carries the blood to the heart. In 

 the frog there are two portal systems, one supplying the kidneys, 

 and the other the liver. 



