CONTROL OF THE CIRCULATION'. 69 



upon the skin of the hand. The part becomes pale, as 

 the arteries have become narrowed. If this action be con- 

 tinued, there may set in a decided reaction, and the part 

 become more red than usual, when the reaction has 

 widened the artery more than it was 

 before the constriction. 



Effect of Exercise on the Size of the 

 Arteries. As there is only a certain 

 amount of blood in the body, it is evi- 

 dent that if one organ receives 

 an extra supply, some other Sym p at hetic 

 organ or organs must, for the Ganglions 

 time, receive less. For in- 

 stance, one begins to walk vigorously. 

 The large muscles of the lower limbs 

 and trunk become active, and they need 

 more blood. They therefore send mes- 

 sages to some nerve center (probably in 

 the spinal cord), and by reflex action the 

 arteries supplying the lower limbs are 

 widened, and these muscles receive more 

 blood. But these muscles make up a 

 very considerable part of the weight and 

 bulk of the body. While in action they 

 take the lion's share of the blood. The 

 brain, at such a time, would receive less, Fig 33 . ventral view 

 and it would be folly to expect the brain of s P inal Cord with 



. ,", . i-ii Sympathetic Gang- 



to work at its full capacity while the u on s of one side 

 blood was called away to other organs. 



Regulation of the Effects of Exercise. When we ex- 

 ercise vigorously, the heart beats faster, and this of itself 

 would tend to increase the blood supply to all organs. 



