MAINTAINING FAKM ANIMALS 51 



some of the body nutrients are oxidized, or slowly burned, with the 

 result that heat is formed. Unlike the burning of fuel in a stove, 

 the oxidations in the body take place at a comparatively low tem- 

 perature. As a result of these oxidations, where there were before 

 glucose, fats, and proteins in the tissues, there now remain carbonic 

 acid gas, water, and urea. The latter is the form in which the nitrog- 

 enous waste of the body, resulting from the breaking down of protein, 

 is chiefly excreted. 



As shown in the preceding chapter, all the energy used up in the 

 various forms of internal work of the body is finally changed to heat. 

 The this energy is lost so far as useful production is concerned, the 

 heat formed helps to maintain the body temperature. The amount of 

 heat so produced is considerable. Even with such an easily digested 

 feed as corn, over one-third of the total energy which the digestible 

 nutrients furnish is converted into heat in the work of masticating, 

 digesting, and utilizing it. With roughages like hay and straw the 

 proportion is much larger. However, in the case of animals exercis- 

 ing normally the larger part of the body heat is produced in the 

 muscular tissues, since all muscular contraction is caused by the oxida- 

 tion, or burning, of nutrients in the muscles. Even when the 

 muscles are not actively contracting, some heat is being generated in 

 them. 



Heat regulation. — Not only must heat be continuously produced in 

 the body, but the temperature must be kept constant under varying 

 external conditions and with supplies of food differing from day to 

 day in amount and heat producing power. This is done by the 

 unconscious regulation of both the production and the loss of heat. 

 The production of heat is governed by decreasing or increasing the 

 oxidations going on in the body tissues. On cold days, for example, 

 we are inclined to eat more heartily and walk more briskly than in 

 warm weather. When chilled, there is also an involuntary rise in 

 heat production, brought about thru a "shivering" of the muscles. 

 The loss of heat from the body is regulated in part by varying the 

 circulation of the blood near the surface of the body. When the 

 temperature of the body is too high, more blood is pumped to the 

 surface, where some of the heat passes off into the air. The produc- 

 tion of sweat and the giving off of water vapor from the lungs are 

 also important means of governing the loss of heat. In addition, the 

 clothing of man and the thick skin, hair, wool, and feathers of animals 

 prevent too rapid loss of heat. 



Heat and energy required for maintenance. — In maintaining a 

 mature animal at rest a certain amount of net energy must be fur- 

 nished by the feed to carry on the internal work of the body. How- 



