640 TEMPERATURE [CH. XLIII. 



Quite otherwise is it in climates like that of the coast-line of 

 British East Africa, where the tropical sun is combined with the 

 moisture-laden wind. There the possibilities of heat loss both by 

 radiation and by evaporation are small, and the English official per- 

 force reduces his heat production to a minimum. He lives indoors, 

 takes as little exercise as possible, and the pallor of his countenance 

 contrasts strongly with the high colour which his colleague in 

 India or Egypt exhibits. 



Certain Factors which govern the Relation between Heat 

 Production and Heat Loss. 



(1) Size. The heat production of the body, other things being 

 equal, depends upon the mass of the body; the heat loss, on the 

 surface. The production therefore varies with the cube of the linear 

 dimensions, whilst the loss of heat only varies with the square. 



The smaller the animal the greater must be its heat production 

 relatively to its heat loss. The loss of heat is diminished both by 

 the occurrence of fur and by the absence of sweat in the skins of 

 most small animals, and the smaller the animal the greater is its 

 metabolism per gramme (see also p. 399). 



The same is, no doubt, true of individuals ; but in this, as in other 

 cases, the natural conditions may be much modified by artificial ones, 

 such as clothing. 



(2) Age. Inasmuch as the young are small, active, and growing, 

 their heat production is relatively large; and further, since the 

 extreme constancy of temperature which an adult man has attained 

 is an evolved characteristic, very young children, in common with 

 animals, are subject to changes of body-temperature which would be 

 of much graver import in older people. Warm-blooded animals in 

 the embryonic stage are practically cold-blooded, the regulatory 

 mechanism which keeps the body-temperature constant not being 

 fully developed at this stage. 



(3) Constitution. Different individuals differ greatly in their 

 power of heat loss. Apart from differences in size and in the faculty 

 of perspiration, there remains such differences as those of compactness 

 of shape, and especially in the amount of adipose tissue with which 

 the viscera are protected. 



The Influence of the Central Nervous System on Heat Regulation. 

 The central nervous system controls the loss of heat directly 

 through the vaso-motor and secretory nerves supplying the skin. 

 That the control of heat production is important, is shown by the 

 effect on the body-temperature of cutting the spinal cord, or of the 

 drug curare. Curare cuts off the muscles from the stimuli which 

 would naturally reach them through the motor nerves. Not only 



