696 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



blood may increase in temperature over that of the arterial blood ; such 

 a state of affairs is seen in the blood coining from the contracting muscles 

 or from a secreting gland. The blood by its continuous circulation 

 through the body tends to equalize the body temperature, giving up 

 heat to tissues which are cooler than itself and withdrawing heat from 

 those which are warmer. The mean between the highest and lowest 

 temperature of the animal body is spoken of as the body temperature, 

 and is generally represented by the temperature taken in the mouth or in 

 the rectum. 



The following figures represent the mean average temperatures of 

 the different domestic animals : 



While these figures represent the average body temperature, varia- 

 tions within narrow limits may often be observed even in perfectly 

 healthy individuals. A variation of one degree or more indicates some 

 failure in the organism or some departure from the natural process of 

 metabolism. It is, therefore, evident that the mechanisms which regulate 

 the balance between the production and loss of heat must be extremely 

 sensitive. Such a regulating mechanism will prevent an increase of the 

 body temperature either by diminishing the production or increasing 

 dissipation of heat, or, in the other case, by increasing the production 

 and diminishing the loss. 



Heat, as already indicated, is lost to the body by conduction to the 

 ingesta and egesta, to the expired air, and by conduction and radiation 

 from the skin and through the evaporation of fluid from the surface of 

 the body. The relative amounts of heat lost by these different channels 

 have been calculated by Helmholtz as follows : through the expired air, 

 5.2 per cent.; through the water of respiration, 14.7 per cent. ; through 

 the skin, 77.5 per cent. The chief means, therefore, of heat dissipation 

 are through the lungs and skin. The more rapid the respiration the 

 greater will be the loss of heat, and in animals which do not perspire the 

 lungs will represent the main source of heat dissipation. In other 

 animals the skin is, no doubt, the great regulator of the body temper- 

 ture. The more blood passes through the cutaneous vessels, the greater 

 will be the loss of heat through radiation, the greater will be the cuta- 



