188 



THE CIRCULATORY APPARATUS 



FIG. 74 



supply. They are located in the tunica media or 

 middle coat. The lining coat or intima of an artery 

 is nourished by the column of blood passing over it. 



The arteries give off branches which become smaller 

 as their destination the capillary area of the cir- 

 culation is reached. Just before 

 emerging into capillaries, these small 

 branches are termed arterioles. 



The Capillaries. The capillaries 

 consist of a series of minute blood- 

 vessels which divide and subdivide 

 to form interlacing net-works. The 

 walls of a capillary consist of a single 

 layer of flattened endothelial cells, 

 nucleated, and held together by a 

 cement substance. These thin-walled 

 vessels possess irritability, contrac- 

 tility, and tonicity. They freely 

 communicate with one another, and 

 are the connecting link for the pas- 

 sage of the blood from the arteries 

 to the veins. They are just large 

 enough to transmit a small red cell. 

 The functions of the capillary wall 

 is to allow an exchange of the nutri- 

 tive elements of the blood with the 

 tissues, and to receive from the tissue 

 the waste products arising as a result 

 of metabolism, taking place in the 

 protoplasm of the tissue cells. This 

 interchange between the blood and the tissues is 

 due to the thinness of the capillary wall, aided by 

 the phenomena of diffusion, osmosis, and infiltration 

 (see Absorption, page 286). 



The Veins. The veins consist of three coats: the 

 tunica intima, media, and adventitia. They differ 

 from the arteries by their middle coat possessing 

 less elastic and muscular tissue, but an increase in the 



Capillaries from the 

 mesentery of a guinea- 

 pig after treatment with 

 a solution of nitrate of 

 silver: a, cells; b, their 

 nuclei. (Gray.) 



