464 ANIMAL HEAT. [BOOK 11. 



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. 



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. And in such animals as the 

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

 important means of regulating the., temperature. The changes 

 which give rise to this loss take place before the inspired air 

 reaches the pulmonary alveoli ; both the warming and the evapo- 

 ration being effected in the nasal and pharyngeal, and to some 

 extent in the bronchial passages. Some observers have maintained 

 that the left side of the heart is warmer than the right, and hence 

 argued that chemical changes leading to a considerable develop- 

 ment of heat take place in the pulmonary capillaries. It would 

 appear however that the right ventricle, owing to its lying 

 nearer to the liver, the high temperature of which has already 

 been mentioned, is in reality rather hotter than the left. And 

 indeed we have no satisfactory evidence of any large amount of 

 heat being produced by any pulmonary metabolism. 



The great regulator however is undoubtedly the skin. The 

 more blood passes through the skin the greater will be the loss of 

 heat by conduction, radiation, and evaporation. Hence, any action 

 of the vaso-motor mechanism which, by causing dilation of the 

 cutaneous vascular areas, leads to a larger flow of blood through 

 the skin, will tend to cool the body ; and conversely, any vaso- 

 motor action which, by constricting the cutaneous vascular areas, 

 or by dilating the splanchnic vascular areas, causes a smaller flow 

 through the skin, and a larger flow of blood through the abdominal 

 viscera, will tend to heat the body. Besides this the special 

 nerves of perspiration will act directly as regulators of temperature, 

 increasing the loss of heat when they promote, and lessening the 

 loss when they cease to promote, the secretion of the skin. The 

 working of this heat-regulating mechanism is well seen in the case 

 of exercise. Since every muscular contraction gives rise to heat, 

 exercise must increase for the time being the production of heat ; 

 yet the bodily temperature rarely rises so much as a degree centi- 

 grade, if at all. By the exercise the respiration is quickened, and 



