EXTERNAL TEMPERATURE. 



69 



these ages we have few satisfactory determinations of the mainte- 

 nance requirement. The results upon swine cited on previous pages 

 seem, it is true, to indicate the contrary relation, viz, a lower relative 

 maintenance requirement for young animals. These results, how- 

 ever, are based upon live-weight experiments and, as already noted, 

 are possibly lower than the true maintenance ration. 



If it be true that the maintenance rations of young animals are 

 relatively greater than those of older ones, we may fairly presume it 

 to be due to a considerable extent to the greater amount of muscular 

 activity usually exhibited by young animals. 



EXTERNAL TEMPERATURE. 



Farm animals belong to that general class known as warm-blooded 

 or homoiothermic animals, whose bodies maintain a nearly constant 

 temperature during health, regardless of that of their surroundings 

 unless the latter be extreme, in which case death soon results. 



REGULATION OF BODY TEMPERATURE. 



Obviously, the regulating mechanism which maintains a constant 

 temperature in spite of variations in the heat production of the body 

 and in the temperature of its surroundings must be very efficient 

 and very exactly adjusted. The regulation is effected in general in 

 two ways, which may be called, respectively, physical and chemical 

 regulation. 



The heat of an animal escapes from the surface of tne body chiefly 

 through the skin, but to some extent also through the air passages, 

 being removed both by conduction, by radiation, and by the evapora- 

 tion of water. A rise of external temperature tends to check the out- 

 flow of heat exactly as it would in the case of an inanimate body. 

 This tendency is compensated by a nervous reflex, which allows the 

 capillary blood vessels of the skin to enlarge so that more blood flows 

 through them, thus tending to raise the temperature of the surface 

 and increase the outflow of heat. This phenomenon is readily ob- 

 served in the flush which follows exposure to high temperatures. This 

 method of regulation is analogous to opening the windows of a room 

 to cool it. If the external temperature continues to rise, perspiration 

 appears, or in the case of animals that have no sweat glands, like the 

 dog, a peculiar form of breathing sets in, and relatively large amounts 

 of water are evaporated from the skin or from the tongue and the in- 

 terior of the mouth and throat. In this way large quantities of heat 

 are carried off as the latent heat of evaporation of water, somewhat as 

 an overheated room may be cooled by sprinkling the floor. When the 

 external temperature falls again, the process is reversed. Sensible 

 perspiration decreases and the blood is diverted from the capillaries 

 of the skin to the internal capillaries. If this happens too quickly, it 



