Chap, v.] NUTRITION. 643 



rise to heat, exercise must increase for the time being the pro- 

 duction of heat ; yet the bodily temperature rarely rises so 

 much as a degree centigrade, if at all. By exercise the respi- 

 ration 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, suffi- 

 cient to neutralize the addition caused by the muscular con- 

 traction, 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 application of external cold or heat defeats 

 its own ends, either partially or completely. Under the influ- 

 ence of external cold the cutaneous vessels are constricted, and 

 the splanchnic vascular areas dilated, so that the blood is with- 

 drawn 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, two observers 

 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. 



§ 427. The production of heat, its variations and regulation. 

 As we have already said the exact determination of the amount 

 of heat produced in the living body is attended with great 

 difficulties ; still certain conclusions have been arrived at based 

 partly on direct calorimetric observations, the more recent ones 

 with improved calorimeters being especially valuable, and partly 

 on what seem to be trustworthy deductions from observed chem- 

 ical changes. 



The rate of production of heat in a living body is deter- 

 mined by a variety of circumstances. In the first place what 



