332 



THE HUMAN BODY 



traced without the aid of a microscope. The smallest arteries are 

 called artenoles. These pass into the capillaries, the walls of 

 which are simpler than those of the arterioles, and which form very 

 close networks in nearly all parts of the Body; their immense num- 

 ber compensating for their small size. The average diameter of a 

 capillary vessel is .016 mm. (i^j inch) so that only two or three 



FIG. 105. A small portion of the capillary network as seen in the frog's web 

 when magnified about 25 diameters, a, a small artery feeding the capillaries; 

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



blood-corpuscles can pass through it abreast, and in many parts 

 they are so close that a pin's point could not be inserted between 

 two of them (Fig. 105). 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 networks 

 and like the last arteries are very small. They soon increase in size 

 by union, and so form larger and larger trunks. These in 



