642 REGULATION OF LOSS OF HEAT. [BOOK n. 



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 

 regulate 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 alveoli ; both the warming and the 

 evaporation are 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 have argued that chemical changes leading to 

 a considerable development 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 ; and 

 this has a more or less double action. In the first place it reg- 

 ulates the loss of heat by means of the vaso-motor mechanism. 

 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 dila- 

 tion of the cutaneous vascular areas, leads to a larger flow of 

 blood through the skin, will tend to cool the body ; and con- 

 versely, any vaso-motor action which, by constricting the cuta- 

 neous 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. In the second place, 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 



