238 ADVANCED PHYSIOLOGY 



by temperatures markedly lower or higher than this; and it 

 follows, therefore, that the greatest vigor is possible only 

 when the body is warm. 



Not only is the body warm but it is kept at an almost uni- 

 form temperature. This varies slightly in different persons, 

 and in the same person different parts of the body show slight 

 variations from the average of 98.6 F. The internal organs 

 are considerably warmer than those on the surface; the tem- 

 perature of the skin itself is not much over 90 F., while that 

 of the liver may be as high as 107 F. Moreover, the body 

 heat differs a little from hour to hour, being greatest between 

 1 P. M. and 4 P. M. and least between 1 A. M. and 

 4 A. M. 



Health is so dependent upon the maintenance of proper 

 body temperature that if there is much departure from the 

 normal, illness results. If the temperature rises to over 

 100 F. we say that the individual concerned has a fever; a 

 slight fall of temperature below 98 F. is an equally sure 

 indication of illness, though we have no special name for this 

 condition and less commonly hear about it. During a high 

 fever a person's temperature may rise to 106 F. or even a 

 little higher, and recovery take place, but a rise to 107 F. 

 or 108 F. is usually followed by death. 



The source of all body heat is the oxidation of foods. All body 

 activity involves the oxidation of food or tissues, and 

 thus is necessarily accompanied by heat production. This 

 explains why we get warm when exercising vigorously, why 

 we like to move around quickly on a cold day, and why we 

 need extra clothing when not exercising. We must not, how- 

 ever, confuse feeling cold with being cold. On a cold winter day 

 one feels much colder than on a warm summer one, and it is 

 hard to believe that the body temperature is practically the 

 same in the two instances. One's feeling of warmth is in 

 the skin; if the blood vessels in the skin are open so that 

 much warm blood can flow through them, one feels warm; 



