THE CIRCULATION 51 



coat which is continuous with the lining of the arteries 

 and veins. This coat is formed of a single layer of 



thin, fiat cells placed edge to edge ____^__ 



(Fig. 22). With a few exceptions, the 

 capillaries are found in great abundance 



in all parts of the body. 



_, . , ,. f. . . ~ FIG. 22. Surface 



Functions of the Capillaries. On of capi u ary highly 



account of the thinness of their walls, magnified, showing its 

 the capillaries are able to serve a two- c at ** thin f lls 



placed edge to edge. 



told purpose in the body : 



1. They admit materials into the blood vessels. 



2. They allow materials to pass from the blood vessels 

 to the surrounding tissues. 



When it is remembered that the blood, as blood, does 

 not escape from the blood vessels under normal conditions, 

 the importance of the work of the capillaries is apparent. 

 To serve its purpose as a carrier, there must be places 

 where the blood can load up with the materials which it is 

 to carry, and places also where these can be unloaded. 

 Such places are supplied by the capillaries. 



The capillaries also serve the purpose of spreading the 

 blood out and of bringing it very near the individual cells 

 in all parts of the body (Fig. 21). 



Functions of Arteries and Veins. While the capillaries provide the 

 means whereby materials may both enter and leave the blood, the 

 arteries and veins serve the general purpose of passing the blood from 

 one set of capillaries to another. Since pressure is necessary for moving 

 the blood, these tubes must connect with the source of the pressure, 

 which is the heart. In the arteries and veins the blood neither receives 

 nor gives up material, but having received or given up material at one 

 set of capillaries, it is then pushed through these tubes to where it can 

 serve a similar purpose in another set of capillaries (Fig. 23). 



Divisions of the Circulation. Man, in common with all 

 warm-blooded animals, has a double circulation, a fact 



