200 



CIKGULATION 



lected into veins by which the blood is returned to the right 

 auricle of the heart. 



The blood supplied to the head by the carotids is re- 

 turned to the heart by two large veins (the jugulars) pass- 

 ing down each side of the neck; while from the arms come 

 two other veins. These four veins finally unite to form 

 the superior vena cava which opens into the right auricle 

 of the heart. In a similar manner various veins return the 

 blood from the lower part of the body and unite in one 

 common vein (the inferior vena cava) which also opens into 

 the right auricle of the heart It is this lower part of the 

 venous system wMch collects the food from the digestive 

 system. 



The portal circulation. — The veins which are found in 

 the walls of the stomach and intestine do not convey the 



blood directly to the in- 

 ferior vena cava. After 

 collecting the food from 

 these organs these veins 

 are collected into a single 

 large vein (the portal 

 vein) which passes di- 

 rectly to the hver. Here 

 this vein splits up into 

 capillaries again, which 

 in turn collect again into a second vein (the Jiepatic 

 vein). This latter pours its blood into the inferior- vena 

 cava. It is to be noted in this case that the capil- 

 laries of the Uver arise from and unite again into veins, 

 and that the food is thus forced to pass through two sepa- 

 rate sets of capillaries before entering the heart. This 

 branching of the portal vein into capillaries is the only 



Fig. 76 — Diagram of the portal circulation. 



