1174 PHYSIOLOGY 



with voluntary exercise to keep ourselves warm, are indications of the 

 important part played by the muscles in determining the heat production 

 of the body. As a rule the immersion of a man in a cold bath for a minute 

 or two increases considerably his output of C0 2 . It is possible, however, 

 to sit in a bath and by an act of the will keep all the muscles in a state of 

 relaxation. Under these circumstances the temperature of the body 

 rapidly falls, and with it the rate of metabolism, as judged by the output of 

 carbon dioxide. 



This process of adjustment of the body temperature by variations in the 

 heat production, so long as it represents the only method, is extravagant of 

 energy directly the difference in temperature between animal and environ- 

 ment 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 con- 

 stant 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 



