THE CIRCULATION 95 



furnished by the heart. What, then, is the purpose of the 

 muscular layer in- the arterial wall? 



156. Use of the Muscular Coat. The body of an adult 

 contains about five quarts of blood. We have learned that 

 the blood supplies the substances needed for the activity of 

 each organ. If an organ is working, it needs more blood 

 than usual, which is supplied by the other organs that are 

 at rest ; they get along with less blood for the time. The 

 muscular coat of the blood vessels makes this possible. 

 This coat is usually in a condition of slight contraction, 

 but the nerves controlling the muscular coat in the blood 

 tubes of the active organs may cease to act, thus allowing 

 the muscular coat to relax and the blood tubes to enlarge 

 under the pressure from the heart, so that the active organs 

 may obtain the additional supply of blood needed. While 

 this is happening, part of the pressure in the blood tubes of 

 the inactive organs is relieved and they become smaller. If 

 cold air strikes the face, the nerves stimulate the muscular 

 coat of the blood tubes in the face to contract more strongly 

 than usual, and the face turns white. This driving of the 

 warm blood from the face saves heat to the body, which 

 would be lost if the warm blood remained in the skin. 

 Thus the amount of blood circulating in any organ is regu- 

 lated by means of the muscular coat of the blood vessels 

 and of the action of the nerves upon this coat. 



157. The Use of the Inner Coat We learned that the 

 inner coat of the heart and blood vessels is made of epi- 

 thelial tissue, like that which forms the outer layer of the 

 skin, and the smooth lining of the mouth and other organs. 

 This lining membrane is very smooth and thus friction is 

 lessened. The friction, however, is inconsiderable in the 

 large vessels ; but in the smaller vessels it is greater ; and in 

 the minute capillaries it becomes of very great importance. 

 We see, therefore, why it is necessary to have this smooth 

 coat in the capillaries, although the muscular and connective 



