THERMAL PHENOMENA OF THE BODY 201 



the internal organs, the smaller will be the amount of heat 

 transferred from the surface of the body to surrounding 

 objects. The student now understands the reason for the 

 reactions of the circulation to changes of surrounding tem- 

 perature. The entire vasomotor mechanism with its vaso- 

 constrictor and vasodilator nerves thus forms part of the 

 mechanism of temperature regulation. The rate of heat loss 

 may also be changed (2) by producing a secretion of per- 

 spiration. This secretion begins at about 68 or 70 F. in 

 the body at rest and increases in amount as the external 

 temperature rises. The sweat glands are thrown into action 

 by nervous impulses. Hence the nervous system through its 

 nerves to the arterioles and the sweat glands controls the 

 output of heat from the body. 



The nervous system also controls the rate of heat produc- 

 tion, for this is changed by increasing or diminishing the 

 activity of the skeletal muscles. We are more active on cold 

 than on warm days, and this apart from any conscious adjust- 

 ment of muscular activity to the temperature needs of the 

 body. We shall return to several interesting features of this 

 part of our subject in later paragraphs. 



16. Reactions of the body at rest and lightly clad to changes 

 of external temperature. Having learned the more important 

 principles concerned in maintaining the constant heat balance, 

 let us now observe the actual behavior of the body as the 

 external temperature changes, assuming that the air remains 

 of moderate humidity and that there is little or no wind. 1 

 To do this let us suppose that the body at rest and lightly 

 clad is exposed, to begin with, to a temperature of 90 F. 

 At this point but little heat is transferred by conduction, con- 

 vection, and radiation from the skin to surrounding objects, 

 since both are so nearly of the same temperature. Hence the 

 main reliance for getting rid of the heat constantly being 

 liberated is upon the evaporation of the perspiration, which 

 1 Consult Fig. 87 when reading this section. 



