848 THE CONSTANT BODILY TEMPERATURE. [BOOK n. 



This mean bodily temperature of warm-blooded animals is, 

 during health, maintained, with slight variations of which we shall 

 presently speak, within a very narrow margin, a rise or indeed a 

 fall of much more than a degree above or below the limit given 

 above being indicative of some failure in the organism, or of some 

 unusual influence being at work. It is evident, therefore, that the 

 mechanisms which co-ordinate the loss with the production of heat 

 must be exceedingly sensitive. It is obvious, moreover, that these 

 mechanisms may act when the bodily temperature is tending to 

 rise, by either checking the production or by augmenting the loss 

 of heat; conversely when the bodily temperature is tending to 

 fall, they may act by either increasing the production or by 

 diminishing the loss of heat. As the regulation of tempera- 

 ture by variations in the loss of heat is better known than 

 regulation by variations in production, it will be best to consider 

 this first. 



532. Regulation by variations in loss. Heat is lost to the 

 body by the warming of the faeces and of the urine, by the warming 

 of the expired air, by the evaporation of the water of respiration, 

 by conduction and radiation from the skin, and by the evaporation 

 of the water of perspiration. It has been calculated that the 

 relative amounts of the loss by these several channels are as 

 follows : In warming the faeces and urine about 3, or according to 

 others 6 per cent. By respiration about 20, or according to others 

 about 9 only per cent., leaving 77, or alternatively 85, per cent, for 

 conduction and radiation and evaporation by the skin. If we 

 attempt to distinguish between the heat which is lost by radiation 

 and conduction from the skin and that which is lost by evapora- 

 tion we find that the two vary so independently and often so 

 conversely under different circumstances that it is useless to attempt 

 to make a general statement as to their proportion. That pro- 

 portion for instance is wholly different when the body is exposed 

 to a hot dry atmosphere and is sweating profusely from what it is 

 when the body is exposed to a cool atmosphere saturated with 

 moisture ; the former exalts the loss by evaporation, the latter that 

 by conduction and radiation. And the like may be said of the loss 

 by the lungs. 



The two chief means of loss then, which are at all susceptible 

 of any great amount of variation, and which can be used to regu- 

 late the temperature of the body, are the skin and the lungs. 



The more air passes in and out of the lungs in a given time, 

 the greater will be the loss in warming the expired air, and in 

 evaporating the water of respiration. In such animals as the 

 dog, which do not perspire freely by the skin, respiration is a most 

 important means of regulating the temperature ; and in the dog a 

 very close connection may be observed between the production of 

 heat and respiratory activity. The changes which give rise to this 

 loss take place before the inspired air reaches the pulmonary 



