850 REGULATION OF LOSS OF HEAT. [BOOK n. 



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. 



533. 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 diffi- 

 culties ; still certain conclusions have been arrived at based partly 

 on direct calorimetric observations, the more recent ones with im- 

 proved calorimeters being especially valuable, and partly on what 

 seem to be trustworthy deductions from observed chemical changes. 



The rate of production of heat in a living body is determined by 

 a variety of circumstances. In the first place what may be called 

 the general rate of metabolism, and so of the production of heat, 

 varies in different kinds of animals. Of two animals of the same 

 bulk and weight placed under the same circumstances one ' lives 

 faster' than the other, metabolizes its living substance more rapidly, 

 and so produces heat more rapidly. Thus direct calorimetric 

 observations, so far as they at present go, shew that a man on the 

 average produces more heat, per kilo, per hour, than does a dog, 

 and a dog more than a rabbit. Probably every species has what 

 may be called its specific coefficient, and every individual his 

 personal coefficient of heat-production, the coefficient being the 

 expression of the inborn qualities proper to the living substance 

 of the species and of the individual. 



A larger living body will naturally produce more heat than a 

 smaller living body of the same nature, since the larger body 

 possesses so to speak a greater number of heat-producing units. 

 But this is neutralized by an opposing tendency. The smaller 

 body, having relatively to its bulk a larger amount of surface, loses 

 heat at a more rapid rate than does the larger body; and therefore, 

 to maintain the balance between loss and production, so as to 

 secure the same constant bodily temperature (and as we have just 

 seen the bodily temperature of warm-blooded animals is remark- 

 ably uniform), it must produce heat, per unit of its body, at a more 



