466 ANIMAL HEAT 



unless both external warmth and artificial respiration be employed. Rabbits 

 not cooled below 25 C. (77 F.) recover by external warmth alone. 



Loss of Heat from the Lungs. The lungs and air-passages are very 

 inferior to the skin as a means for lowering the temperature. In giving 

 heat to the air breathed, the lungs stand next to the skin in importance. As 

 a regulating power, the inferiority is very marked. The air which is ex- 

 pelled from the lungs leaves the body at about the temperature of the blood, 

 and is always 'saturated with moisture. No inverse proportion, therefore, 

 exists, as in the case of the skin, between the loss of heat by radiation and 

 conduction, on the one hand, and by evaporation, on the other. The colder 

 the air and the drier, for example, the greater will be the loss in all ways. 

 Neither is the quantity of blood which is exposed to the cooling influence of 

 the air diminished or increased in the lungs, so far as is known, in accord- 

 ance with any need in relation to temperature. It is true that by varying the 

 number and depth of the respirations, the quantity of heat given off by the 

 lungs may be made to vary also for a few minutes. But the respiratory 

 passages, while they must be considered important means by which heat 

 is lost, are altogether subordinate, in the power of actively regulating the 

 temperature. 



The loss of heat used to warm foods is an obvious method of expenditure 

 of heat which may be resorted to, especially in certain fevers. The loss of 

 heat by the excreta discharged from the body at a high temperature must be 

 of no use as a means of regulating the temperature, since the amount so lost 

 must be capable of little variation. 



Variation in the Production of Heat. It may seem to have been 

 assumed, in the foregoing pages, that the only regulating apparatus for tem- 

 perature required by the human body is one that shall, more or less, produce 

 a cooling effect; as if the amount of heat produced were always, therefore, 

 in excess of that which is required. Such an assumption would be incorrect. 

 The body has the power of regulating the production of heat, as well as its 

 loss. 



The production of heat in the body is apparently different for each ani- 

 mal; i.e., the absolute amount of heat set free by different animals in a given 

 period varies. Each individual has his own coefficient of heat production. 

 From all that has been said on the subject it will be seen that the amount of 

 heat for all practical purposes depends upon the metabolism of the tissues of 

 the body; everything, therefore, which increases that metabolism will increase 

 the heat production; so, therefore, the absolute amount of heat produced by a 

 large animal, having a larger amount of tissues in which metabolism may 

 go on, will be, ceteris paribus, greater than that of a small animal. But the 

 activity of the tissue change in a small animal may be greater than in a large 

 one, as measured per kilo of body weight, and naturally no strict line can be 

 drawn between the two. 



