670 ANIMAL HEAT 



reaction occurs after the person comes out of the bath. The rectal 

 temperature falls to a minimum, which is reached in twenty to 

 thirty minutes after exit from the bath, and then gradually returns 

 to normal. This fall of internal temperature is due to the heating 

 of the superficial portions of the body at the expense of the central 

 portions. By training, the fall of temperature is greatly lessened, 

 the heat-regulating mechanism acquiring, so to speak, with practice, 

 greater promptitude and precision of adjustment. 



It must be admitted, then, that especially in the smaller homoio- 

 thermal animals the metabolic changes normally going on in the 

 resting muscles may be reflexly increased without the usual accom- 

 paniment of mechanical contraction,* and that such an increase of 

 ' chemical tone ' is an important means by which the temperature 

 is regulated. It is possible that other organs besides the muscles 

 may be concerned, though not to a sufficient extent to secure the 

 due regulation of temperature during curara paralysis. It is 

 obvious that in man, whose environment is so much under his own 

 control, a mere automatic regulation is less required than in the 

 inferior animals, and that a regulative power, if present in rudiment, 

 would tend to ' atrophy ' by disuse, or, at all events, to become less 

 sensitive to slight changes of temperature. In the larger animals, 

 again, mere bulk is an important safeguard against any sudden 

 change of internal temperature. To reduce the temperature of a 

 horse or an elephant by i, a considerable quantity of heat must 

 be lost, while a very slight loss would suffice to cool a mouse by 

 that amount. Not only so, but the surface by which heat is lost is 

 greater in proportion to the mass of the body in small than in large 

 animals. The power of rapidly increasing the heat-production to 

 meet a sudden demand is, therefore, far more important to the 

 mouse than to the horse; and the fact (p. 616) that the metabolism 

 of an animal varies approximately as its surface, and not as its 

 mass,f is an illustration of the nice adjustment by which heat- 

 equilibrium is maintained. There is reason to believe that at a 

 temperature equal to that of the human body the heat- pro duct ion 

 of a frog per unit of mass would equal that of a man or other large 

 mammal, although it would be far less than that of a small homoio- 



* Increased tonus of the muscles might, however, account for a portion 

 of the increased heat-production. 



t The relation between mass (M) and surface (S) in man is approximately 



o 3/-S/T 



given by the equation ^ = K, and the relation between surface, mass, 

 length of body (L), and circumference of chest (C) just above the nipples in 



Q 6/"vr4 T 4 /~*2 



the ' mean ' position of respiration, by the equation V J^ ' ' = K'. M is 



expressed in grammes, S in square centimetres, L and C in centimetres. K is 

 - a constant whose mean value is 12-3, and K' a constant whose mean value is 

 4-5 (Meeh). 



