CHAP, v.] NUTRITION. 811 



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 

 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 

 regulates 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 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. 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 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 centigrade, 

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

 of heat by the lungs increased. The circulation of blood is also 

 quickened, and the cutaneous vascular areas becoming dilated, a 

 larger amount of blood passes through the skin. Added to this, 

 the skin perspires freely. Thus a large amount of heat is lost to 

 the body, sufficient to neutralize the addition caused by the 

 muscular contraction, the increase which the more rapid flow of 



