THE HUMAN BODY, 



that a pin's point could not be inserted between two of 

 them. It is while flowing in these delicate tubes that the- 

 blood does its nutritive work, the arteries being merely 

 supply- tubes for the capillaries. 



The Veins. The first veins arise from the capillary net- 

 works in various organs, and like the last arteries are very 

 small. They soon increase in size by union and so form 



FIG. 82. A small portion of the capillary network as seen in the frog's web 

 when magnified about 2o diameters, a, a small artery feeding the capillaries ; 

 v, v, small veins carrying blood back from the latter. 



larger and larger trunks. These in many places lie near or 

 alongside the main artery of the part, but there are many 

 more large veins just beneath the skin than there are large 

 arteries. This is especially the case in the limbs, the main 

 veins of which are superficial and can in many persons be 



