136 HUMAN PHYSIOLOGY. 



pneumonia; in cholera it falls as low as 80 F. Death usually occurs when 

 the heat remains high and persistent, from 106 to 110 F.; the increase of 

 heat in disease is due to excessive production rather than to diminished 

 elimination. 



The source of heat is to be sought for in the chernic decompositions 

 and hydrations taking place during the general process of nutrition, and 

 in the combustion of the food materials by the oxygen of the inspired air; 

 the amount of its production is in proportion to the activity of the internal 

 changes. 



Every contraction of a muscle, every act of secretion, each exhibition of 

 nerve force, is accompanied by a change in the chemic composition of the 

 tissues and an evolution of heat. The reduction of the disintegrated tissues 

 to their simplest form by oxidation, and the combination of the oxygen of the 

 inspired air with the carbon and hydrogen of the blood and tissues, results 

 in the formation of carbonic acid and water and the liberation of a great 

 amount of heat. 



Certain elements of the food, particularly the carbo-hydrates and the /ate, 

 undergo oxidation without taking part in the formation of the tissues, being 

 transformed into carbon dioxid and water, and thus increase the sum of 

 heat in the body. 



Heat-producing Tissues. All the tissues of the body add to the general 

 amount of heat, according to the degree of their activity. But special struc- 

 tures, on account of their mass and the large amount of blood they receive, 

 are particularly to be regarded as heat producers, e. g. : 



1. During mental activity the brain receives nearly one-fifth of the entire 

 volume of blood, and the venous blood returning from it is charged with 

 waste matters, and its temperature is increased. 



2. The muscular tissue, on account of the many chemic changes occurring 

 during active contractions, must be regarded as the chief heat-producing 

 tissue. 



3. The secreting glands, during their functional activity, add largely to the 

 amount of heat. 



The entire quantity of heat generated within the body has been demon- 

 strated experimentally to be about 2,300 calories, a calory, or heat unit, 

 being that amount of heat required to raise the temperature of one kilogram 

 of water (2.2 pounds) i C. This quantity of heat, if not utilized and retained 

 within the body, would elevate its temperature in twenty-four hours about 

 60 F. That this volume of heat depends very largely upon the oxidation 

 of the food-stuffs can be shown experimentally. 



