THE TEMPERATURE OF THE BODY 1313 



animal and environment attains any considerable degree. In the 

 very perfect adjustment of the temperature which is present in the 

 higher mammals, regulation of the heat loss plays a greater part than 

 regulation of heat production. The economy of adjustment by heat 

 loss is well shown if we compare in echidna and rabbit respectively 

 the percentage alteration in C0 2 production when the difference in 

 temperature between animal and environment varies from 10 C. 

 to 20 C. This is for echidna 72 per cent., for mammals 16 per cent. 

 In echidna variations in heat loss can be practically neglected, so 

 that the whole of the work of regulating the body temperature falls 

 on the heat production. As soon as the external temperature falls 

 below a certain degree the mechanism fails, the animal's temperature 

 falls, and it passes into a state of hibernation. 



THE REGULATION OF HEAT LOSS 



In all temperate climates, and in fact in all climates except under 

 certain exceptional conditions, the temperature of the warm-blooded 

 animal is higher than that of his environment, so that there must be a 

 constant loss of heat from the surface of the body. In the warm- 

 blooded animals in the arctic regions, and in those which have adopted 

 an aquatic existence, the thick layer of fat which underlies the skin 

 protects the active portions of the body, the muscles and internal 

 organs, from excessive loss of heat to the surrounding medium. In 

 most terrestrial animals the loss of heat is also diminished by fur and 

 feathers with which these animals are clothed, and in ourselves the 

 same office is performed by clothes, which are capable of voluntary 

 graduation to external conditions. The value of these different 

 coverings depends on the fact that air is a very bad conductor of heat. 

 In the case of a naked man the layer of air immediately in contact 

 with the surface of the body is warmed, becomes lighter, and rises, its 

 place being taken by fresh cold air. Loss of heat is increased by a 

 draught, which quickens the rate at which the layers of air in contact 

 with the surface are renewed. In the case of clothes the material 

 encloses layers of air and hinders it from free circulation, and it is the 

 air enclosed in the meshes of the garments and between the different 

 layers of garments that plays the greater part in preventing loss of 

 heat. The rapid cooling effect of wet clothes is partly due to the replace- 

 ment of layers of air by water, which is a much better conducting fluid. 

 In addition to the loss of heat by convection, i.e. by warming the 

 layers of air in contact with the body, heat is also lost by radiation, i.e. 

 by an exchange of heat between the surface of the body and that of 

 surrounding cooler objects. This loss is also prevented by clothing. 

 Since the material of which clothes are made does not allow the passage 

 of radiant heat, they absorb the heat leaving the body and become 



83 



