i 2 6 HUMAN PHYSIOLOGY CHAP. 



them to dilate or contract as the supply of blood in 

 different organs may require. 



The arteries divide and subdivide until they are very 

 small and finally end in capillaries, which are the small- 

 est blood vessels of the body. The capillaries permeate 

 every part of the body. You cannot prick yourself 

 with a needle without rupturing some of these little 

 tubes. The walls of the capillaries are very thin, con- 



FIG. 66. Showing the relation of the capillaries to an artery and a vein 

 (Zinns). 



sisting of a single layer of flattened, elongated cells held 

 together by a small amount of cementing material. 

 They are such small vessels that the corpuscles must 

 pass through one by one, and so arranged as to be able 

 to carry blood to each and every living cell of the body. 

 It is from the capillaries that some of the blood escapes 

 through the thin walls and comes in contact with the 

 cells. 



Although the capillaries are numerous in all parts of 

 the body, they are exceedingly short, being only a small 



