456 HANDBOOK OF PHYSIOLOGY. 



ture 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, 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, so far as is 

 known, in accordance 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 some extent, 

 to vary also. But the respiratory passages, while they must be considered 

 important means by which heat is lost, are altogether subordinate, in 

 the power of regulating the temperature, to the skin. 



(c) By Warming Cold Foods. This is an obvious method of expendi- 

 ture of heat which may be resorted to, but the loss of heat by the excreta 

 discharged from the body at a high temperature, must be of little 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 

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

 produce a cooling effect; and as if the amount of heat produced were 

 always, therefore, in excess of that which is required. Such an assump- 

 tion would be incorrect. We have the power of regulating the produc- 

 tion of heat, as well as its loss. 



The regulation of the production of heat in the body is apparently 

 different for each animal, as the absolute amount of heat set free by 

 different animals in a given period varies; in one the production of heat 

 exceeds that in another. It is even said that 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 produc- 

 tion, 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, cceteris paribus, greater than that of a small animal. But of 

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

 than in a large one, and naturally no strict line can be drawn between 

 the two. 



The ingestion of food has been proved to increase the metabolism of 

 the tissues, and so, as one would expect, the rate of heat production is 

 found by experiment upon the dog to be increased after a meal, and 

 in this animal the heat production reaches its height about 6 to 9 houra 

 after a meal. 



