604 ANIMAL HEAT [CH. XL 



Regulation ly Variations in Loss. The two means of loss suscep- 

 tible of any amount of variation are the lungs and the skin. The 

 more air that passes in and out of j the lungs, the greater will be the 

 loss in warming the expired air and in evaporating the water of respira- 

 tion. In such animals as the dog, which perspire but little, respiration 

 is a most important means of regulating the temperature; and in 

 these animals a close connection is observed between the production 

 of heat and the respiratory activity. The panting of a dog when 

 overheated is a familiar instance of this. A dog also, under the same 

 circumstances, puts out its tongue, and loses heat from the evapora- 

 tion that occurs from its surface. The great regulator, however, is 

 undoubtedly the skin, and this 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 conduction, 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 ly Variations in Production. The rate of production 

 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 con- 

 siderations 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 : 



