PHYSIOLOGY OF THE NERVOUS SYSTEM 311 



to cold, the small arteries going to the skin quickly diminish in size, 

 check the flow of blood to the surface, and prevent too great a loss 

 of heat. In this case, impulses starting at the surface of the body are 

 transmitted to the bulb and then through the efferent neurons to the 

 muscles in the walls of the arteries. In a somewhat similar manner, 

 heat leads to a relaxation of the arterial walls and an increase in the 

 blood supply to the skin. Other changes in the blood supply to differ- 

 ent parts of the body are also of the nature of reflex actions. As in the 

 work of digestion, neurons having their cell-bodies in the sympathetic 

 ganglia aid in the control of the circulation. 



Purposes of Reflex Action. The examples of reflex 

 action so far considered illustrate its two main purposes 

 (i) protection, and (2) a means of controlling important 

 processes. 



The pupil has but to study carefully the reflex actions 

 of his own body for a period, say of two or three weeks, 

 in order to be convinced of their protective value. He 

 will observe that portions of his body have, on exposure to 

 danger, been moved to places of safety, while in some 

 instances, like falling, his entire body has been adjusted 

 to new conditions. He will also find that reflex action 

 is quicker, and for that reason offers in some cases better 

 protection, than movements directed by the mind. In 

 digestion and circulation are found the best examples of 

 the control of important processes through reflex action. 



Voluntary Action. It is observed that reflex action, in 

 the sense that it has so far been considered, is not the 

 usual mode of action of the external organs, but is, instead, 

 a kind of emergency action, due to unusual conditions and 

 excitation by strong stimuli. Voluntary actions, on the other 

 hand, represent the ordinary, or normal, action of these 

 organs. They comprise the movements of the body of 

 which we are conscious and which are controlled by the 

 mind. But while they are of a higher order than reflex 



