THE VASCULAR SYSTEM 397 



Stomach, the pyloric vein from the pylorus, pancreas, and duodenum, 

 and the cystic vein from the gall-bladder. 



The common iliac veins parallel the arteries of the same name and are 

 formed by the union of the external and hypogastric or internal iliac veins. 

 The middle sacral vein, which is thought to be a rudiment of the caudal 

 vein of lower vertebrates, returns blood from the sacrum and lower pelvis 

 into the left common iliac vein. The hypogastric vein parallels the artery 

 of the same name. Its tributaries are the hemorrhoidal vein from the 

 rectum, uterovaginal vein, vesical from the bladder, and the pudendal 

 vein from the external genitals. The hypogastric also receives blood 

 from the inferior and superior gluteal, obturator, lateral sacral, and 

 ilio-lumbar veins, the distribution of which is similar to that of the arteries 

 of the same names. 



In the leg the chief superficial vein is the great saphenous which 

 returns to the femoral vein most of the blood from the surface of the 

 thigh, leg, and foot. The veins of the skin beginning with the digital 

 veins of the foot form plantar and dorsal networks which combine on 

 the inner side of the leg into the great saphenous vein. The small 

 saphenous vein extends from the foot along the back of the calf to the 

 knee where it combines with the popliteal vein. 



The deep veins of the leg parallel the arteries and are given correspond- 

 ing names. In the region of the shank there are two veins to each artery. 

 Beginning with the plantar veins in the foot, the more important of the 

 deep veins of the leg are the anterior and posterior tibial veins, which 

 unite in the knee region to form the popliteal vein. In the upper part 

 of the thigh, the popliteal vein in turn becomes the femoral vein which 

 directly continues as the external iliac. 



The Lymphatic System. The circulatory system thus far described 

 is closed, in contrast with the open or lacunar systems of invertebrates. 

 That is to say, the blood vessels of vertebrates form a system of tubes the 

 walls pf which are continuous and without openings. But to nourish 

 the cells in the tissues the blood vessels must be permeable. Loss of 

 fluid plasma from the blood takes place in the capillaries, which are 

 formed of a single layer of endothehum. It is generally thought to be 

 due to three factors, blood pressure, osmosis, and diffusion; but active 

 secretion by the endothelial cells may also be involved. 



The fluid which passes from the capillaries forms lymph. Lymph 

 is essentially blood plasma from which the corpuscles, except a few 

 leucocytes, have been removed. The lymph lost from the blood capil- 

 laries is taken up by the lymphatic capillaries which like the blood capil- 

 laries are microscopic vessels whose walls consist of a single layer of 

 endothehum. Unlike blood capillaries, however, the lymphatics vary 



