152 HUMAN PHYSIOLOGY. 



Certain elements of the food, particularly the non-nitrogenized 

 substances, undergo oxidation without taking part in the formation 

 of the tissues, being transformed into carbonic acid 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 structures, on account of their mass and the large amount 

 of blood they receive, are particularly to be regarded as heat pro- 

 ducers, 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 oc- 

 curring 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 

 demonstrated experimentally to be about 2,300 calories, a calory, 

 or heat unit, being that amount of heat required to raise the tempera- 

 ture 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. 



The normal temperature of the body is maintained by a constant 

 expenditure of the heat in several directions : 



1. In warming the food, drink, and air that are consumed in twenty- 

 four hours. For this purpose about 157 heat units are required. 



2. In evaporating water from the skin and lungs, 619 heat units 

 being utilized for this purpose. 



3. In radiation and conduction. By these processes the body loses 

 at least fifty per cent, of its heat, or 1,156 heat units. 



4. In the production of work ; the work of the circulatory, respiratory, 

 muscular, and nervous apparatus being performed by the trans- 

 formation of 369 heat units into units of work. 



The nervous system influences the production of heat in a part by 

 increasing the amount of blood passing through it by its action upon 

 the vasomotor nerves. Whether there exists a special heat-center 

 has not been satisfactorily determined, though this is probable. 



