CH. XL.] REGULATION OF TEMPERATURE. 6oi 



has a double action. In the first place, it regulates the loss of 

 heat by its vaso-motor mechanism ; the more blood passing 

 through the skin, the greater will be the loss of heat by con- 

 duction, radiation, and evaporation. Conversely, the loss of heat 

 is diminished by anything that lessens the amount of blood in the 

 skin, such as constriction of the cutaneous vessels, or dilatation of 

 the splanchnic vascular area. In the second place, the special nerves 

 of the sweat-glands are called into action. Familiar instances 

 of the action of these two sets of nerves are the reddening of the 

 skin and sweating that occurs after exercise, on a hot day, or in a 

 hot-air or vapour bath, and the pallor of the skin and absence 

 of sensible perspiration on the application of cold to the body. 



Regulation by Variations in Production. The rate of produc- 

 tion of heat in a living body, as determined by calorimetry, 

 depends on a variety of circumstances. It varies in different 

 kinds of animals. The general rate of katabolism of a man is 

 greater than that of a dog, and of a dog greater than that of a 

 rabbit. Probably every species has a specific coefficient, and 

 every individual a personal coefficient of heat production, which 

 is the expression of the inborn qualities proper to the living 

 substance of the species and individual. Another factor is the 

 proportion of the bulk of the animal to its surface area, the 

 struggle for existence raising the specific coefficient of the animals 

 in which the ratio is high. Other important considerations are 

 the relation of the intake of food to metabolic processes, and the 

 amount of muscular work which is performed. These various 

 influences are themselves regulated by the nervous system, and 

 physiologists have long suspected that afferent impulses arising in 

 the skin or elsewhere may, through the central nervous system, 

 originate efferent impulses, the effect of which would be to 

 increase or diminish the metabolism of the muscles and other 

 organs, and by that means increase or diminish respectively the 

 amount of heat there generated. That such a metabolic or 

 thermogenic nervous mechanism does exist in warm-blooded 

 animals is supported by the following experimental evidence : 



(1) Though in cold-blooded animals a rise or fall of the 

 surrounding temperature causes respectively a rise and fall of their 

 metabolic activity, in a warm-blooded animal the effect is just the 

 reverse. Warmth from the exterior demands a diminished pro- 

 duction of heat in the interior, and vice versd. For exceptions, 

 see p. 590. 



(2) That this is due to a reflex nervous impulse is supported 

 by the fact that a warm-blooded animal, when poisoned by curare, 

 no longer manifests its normal behaviour to external heat and cold, 



