CHAPTER XXI 



REGULATION OF THE FLOW OF BLOOD 



316. Vaso-motor nerves. The muscles in the walls of 

 the smaller arteries regulate the amount of blood passing 

 through them. A special set of nerves, called vaso-motor 

 nerves, causes the arteries to contract. When these nerves 

 are paralyzed, the muscles relax, and the artery becomes 

 fully distended by the pressure of the blood. When any 

 part of the body is working, its arteries dilate in order 

 to supply a greater amount of blood to the part. 



The vaso-motor nerves are affected by influences from 

 the brain. Embarrassment and bashfulness paralyze those 

 of the head, so that more blood goes to the face and it 

 becomes redder, or blushes. On the other hand, fear and 

 grief stimulate the nerves and cause a contraction of the 

 arteries, which drives the blood from the face so that pale- 

 ness results. Heat applied to the skin causes the arteries 

 to dilate, and thus to contain more blood. 



317. Congestion. More than the natural quantity of blood re- 

 maining in a part for some time is called congestion. It is liable to 

 injure the cells. Cold causes the arteries of the skin to contract so 

 that 'less blood passes through them, and consequently an extra amount 

 of blood flows through the deeper arteries. So congestion of the deeper 

 parts often results, and the injured cells become unable to resist the 

 growth of disease germs. In this way we often take cold. 



318. Secondary effects of heat and cold. When heat has 

 acted upon the skin for some time it causes a contraction of the blood 

 tubes. When first put into a tub a washerwoman's hands become 



185 



