CHAP. IX.] 



THE VASCULAR SYSTEM. 



103 



Venous return. The arteries begin as large trunks, which 

 gradually become smaller and smaller until they end in the 

 small capillary tubes, while the veins begin as small branches 

 which at first are scarcely distinguishable from the capillaries. 

 These small branches, receiving the blood from the capillaries 

 throughout the body, unite to form 

 larger vessels, and end at last by 

 pouring their contents into the 

 right auricle of the heart through 

 two large trunks, the superior vena 

 cava and the inferior vena cava. 

 The veins, however, which bring 

 back the blood from the stomach, 

 intestines, spleen, and pancreas, 

 do not take the blood directly to 

 the heart, but first join to form 

 a large trunk, the portal vein, 

 and carry this blood to the 

 liver. When the portal vein enters 

 the liver, it breaks up into cap- 

 illaries, which, after branching 

 throughout the liver substance, 

 unite to form the hepatic veins: 

 by them the blood is conveyed 

 into the inferior vena cava. This 

 constitutes what is called the 

 portal circulation, and is the only 

 example in the body of a vein 

 breaking up into capillaries. 



The veins consist of a super- 

 ficial and a deep set, the former 

 running immediately beneath the 

 skin and hence named subcuta- 

 neous, the latter usually aocom- 

 panying the arteries and named 

 venoB comites. These two sets of veins have very frequent com- 

 munications with each other, and the anastomoses of veins 

 are always more numerous than those of arteries. 



The systemic veins that is, all the veins of the body with 

 the exception of the pulmonary and portal veins are naturally 

 divided into two groups. 



FIG. 79. ARTERIES OF THE FOOT. 

 <;, 7, 8, 



