Control of Circulation. 



73 



brought by the sympathetic nerves. Of course the face 

 may turn pale as the result of the stopping, or checking, 

 of the action of the heart, as in ordinary fainting. 



Ordinary Changes in Blood Flow. Without going to the 

 extreme of pallor and blushing, the color of the face varies, 

 under the control of the sympathetic nerves. All the or- 

 gans of the body receive now more, now less, blood, accord- 

 ing as they need it. And all this variation in blood supply 

 is regulated by the sympathetic nervous system. 



Effect of Exercise on" the Size of the Arteries. When 

 the muscles work, of course they need more blood. To 

 give them more blood the 

 arteries widen. When one 

 exercising actively, the 



Symoathetic Nerve Chains 

 GRAY \ 



IATTEP 



IS 



muscles take so much blood 

 that we should not 



Dorsal Root 



expect the brain or spmai Nerve _ 

 the digestive organs Ventral Root 

 to do much work, for there 

 is only a certain amount of 

 blood in the body. Hence if 

 one organ, or set of organs, 



etS more blood, the Other Fig. 46. Relation of Spinal Cord and 



Sympathetic Nervous System. 



organs must, for the time, 



receive less. Therefore we see why we should rest after 



eating, both from muscular as well as from mental work. 



How the Heart is made to Beat Faster. When many 

 large muscles are at work, it is not enough merely to widen 

 the arteries. This would allow more blood to go to them, 

 but would not send them as much as they need. The 

 heart must beat faster, or with more force, or both. And 

 the heart is made to beat faster and stronger by the nerve 



