CHAP, v.] NUTRITION. 465 



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 neutralise the addition caused by the 

 muscular contraction, the increase which the more rapid flow of 

 blood through the abdominal organs might tend to bring about 

 being more than sufficiently counteracted by their smaller supply 

 for the time. The sense of warmth which is felt during exercise 

 in consequence of the flushing of the skin, is in itself a token that 

 a regulative cooling is being carried on. In a similar way the ap- 

 plication of external cold or heat, either partially or completely, 

 defeats its own ends. Under the influence of external" cold the 

 cutaneous vessels are constricted, and the splanchnic vascular 

 areas dilated, so that the blood is withdrawn from the colder and 

 cooling regions to the hotter and heat-producing organs. This 

 vascular change may be used to explain the fact that stripping 

 naked in a cold atmosphere often gives rise to a distinct increase in 

 the mean temperature of the blood, as indicated by a thermometer 

 placed in the mouth, though possibly the effect may be partly 

 due to an actual increase of the production of heat. Under the 

 influence of external warmth, on the other hand, the cutaneous 

 vessels are dilated, a rapid discharge of heat takes place ; and if 

 the circumstances be such that the body can perspire freely, and 

 the perspiration be readily evaporated, the temperature of the 

 body may remain very near to the normal, even in an excessively 

 hot atmosphere. Thus, more than a century ago, Drs Fordyce and 

 Blagden were able to remain with impunity in a chamber heated 

 even. to 127 (260 Fahr.), and with ease in one so hot, that it 

 became painful for them to touch the metal buttons of their 

 clothing. It is unnecessary to give any more examples of this 

 regulation of temperature by variations in the loss of heat ; they 

 all readily explain themselves. 



Regulation by variations in production. It is not however 

 solely by variations in the loss of heat that the constant tempera- 

 ture of the warm-blooded animal is maintained Variations in the 

 amount of heat actually generated in the body constitute an 

 important factor not only in the maintenance of the normal 

 temperature, but also probably in the production of the abnormally 

 high or low temperatures of various diseases. Many considerations 

 have long led physiologists to suspect the existence of a nervous 

 mechanism by which afferent impulses arising in the skin or 

 elsewhere might through the central nervous system originate 

 efferent impulses whose effect would be to increase or diminish the 

 metabolism of the muscles or other organs and thus to increase or 

 diminish the amount of heat generated for the time being in the 

 body. The existence in fact of a metabolic or thermogenic nervous 

 mechanism, comparable in many respects to the vaso-motor 

 p. 30 



