592 A MANUAL OF PHYSIOLOGY 



in the production of heat, so that the axillary temperature fell 

 comparatively little e.g., only i C. during a stay of three 

 hours in a bath at 15 C. With short periods of immersion, a 

 characteristic 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 prompti- 

 tude and precision of adjustment. 



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

 homoiothermal animals the metabolic changes normally going 

 on in the resting muscles may be reflexly increased without the 

 usual accompaniment 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-pro- 

 duction to meet a sudden demand is, therefore, far more im- 

 portant to the mouse than to the horse ; and the fact (p. 549) 

 that the metabolism of an animal varies approximately as its 

 surface, and not as its mass,* is an illustration of the nice adjust- 

 ment by which heat-equilibrium is maintained. 



* The relation between mass (M) and surface (S) in man is approxi- 

 mately 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 the ' mean ' position of respiration, by the equation 



V ' = K'. M is expressed in grammes, S in square centimetres, 



JVj. JL/ O 



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). 



