230 MAITUAL OF CATTLE-FEEDIKG. 



going on in tlie body, like similar changes outside tlie 

 body, give rise to a liberation of beat. Indeed, all tbe 

 force conveyed to tbe body by tbe food leaves it either as 

 motion or heat, all the actions of the internal organs, all 

 the molecular labor of the nervous and other tissues, etc., 

 being finally converted into heat. It has been estimated 

 by eminent authorities that, in man, of the total energy 

 represented by the food, from four-fifths to five-sixths 

 takes the form of heat. This production of heat, of course, 

 implies a corresponding consumption of food-material, 

 just as the production of heat in a stove implies the con- 

 sumption of fuel ; so that it is evident that any change in 

 the amount of heat set free has a direct effect on the de- 

 mands of the body for food and on the results of feeding. 



Vital Heat. — The bodies of Mrarm-blooded animals 

 (birds and mammals) maintain a very constant temperature 

 at all times, in spite of great variations in the tempei^ature 

 of their surroundings. The production of mtal hea% as it 

 is called, by the oxidation of food-elements, and the losses of 

 it to which the body is subject, are so balanced as to result 

 in keeping the temperature of the body at from 95° to 

 104:^ F., a variation of more than a degree or two from the 

 normal temperature of an animal indicating serious dis- 

 turbance of the organism. 



This regulation of the vital heat appears to be effected 

 in two ways : first, by variations in the loss, and second, 

 by variations in the production. The chief sources of loss 

 of heat by the body are : 



1. Conduction and Radiation from the Skin. 



% Evaporation of Water from the Skin and Lungs. 



3. Warming of the Ingesta (Food and Drink). 



These we will take up in their order and consider how 

 in each case the balance of the vital heat is kept up. 



