162 ADVANCED PHYSIOLOGY 



usually a supply of those things for which there is a demand: 

 just so here, the "law of supply and demand" is constantly 

 holding true; blood will flow rapidly and vigorously whenever 

 one does things which demand a plentiful flow. Vice versa, 

 a sluggish flow is the rule where the habits of the person are 

 sluggish. 



This is the reason for the bracing effect produced through 

 the stimulation of the skin by rubbing, as after a bath; for 

 this affects surface capillaries, and their walls relax. The 

 inrush of blood reacts on all the contributing arteries, and a 

 readjustment of many vessels takes place, rousing them 

 from their inert condition. In the same way the skin is stimu- 

 lated by fresh air, with its accompanying changes of tempera- 

 ture, which produce a response in the vaso-motor system 

 creating a mild, though extensive effect on the whole body. 

 One single pursuit, one single kind of activity, mental or 

 physical, makes the supply and demand one-sided. Diver- 

 sity of exercise and interest is necessary to secure "a sound 

 mind in a sound body/' an indispensable requisite, if one is 

 to meet life with thrill and enthusiasm. Exercise, either in 

 work or recreation, produces a demand for new materials, for 

 fresh blood. Change in one's train of thought, as in con- 

 genial, lively conversation, upsets the listlessness of the nerve 

 centers, affects the rate of blood flow, and tones up the whole 

 system. 



The Influence of Heat and Cold upon Circulation. Changes 

 in temperature cause great modification in the activities of all 

 organs in the body. As a rule all living tissues work with 

 greater readiness when warm than when cold. The influence 

 of heat is especially noticed in its effect on the blood vessels 

 in the skin; when the body is warm the walls of the arteries in 

 some areas relax more than usual, the capillaries become 

 over-full and one is said to be "flushed with heat." 



On the other hand cold causes blood vessels to contract, 

 and all the muscles to act slowly. The first effect of going 



