644 REGULATION OF PRODUCTION OF HEAT. [Book ii. 



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, as 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 remarkably uniform), it must produce heat, per unit of its 

 body, at a more rapid rate. As a rule the greater loss of heat 

 owing to the relatively greater surface is so marked that of two 

 animals having the same constant bodily temperature, of two 

 species of mammals, or of two individuals of the same race, we 

 should expect the smaller one to produce a relatively larger 

 amount of heat. And direct calorimetric observations shew 

 that this is so. The struggle for existence has raised what Ave 

 have just called the specific or personal coefficient of the smaller 

 animal. 



From what we have seen concerning the immediate effects 

 of a meal, we should be inclined to expect that food would tem- 

 porarily increase the production of heat; and not only is this 

 view confirmed by common experience and by our own sensa- 

 tions, but direct calorimetric observations afford experimental 

 proof of its truth, hi the dog it has been found that the rate 

 of production increases after a meal, reaching its maximum from 

 the 6th to the 9th hour, and then declining to a level which may 

 be regarded as that secured by the general metabolism of the 

 body, and which appears to be maintained with remarkable con- 

 stancy until after long starvation the economy begins to break 

 down. 



Labour, muscular work, has a powerful influence in increas- 

 ing the production of heat. As we have seen, of the total heat 

 produced in the body, a certain portion must always be attrib- 

 uted to muscular contractions which even in the most quiet body 



