ANIMAL HEAT 589 



32 C., had a much scantier covering. The increased protection 

 against heat-loss in the case of the ' cooled ' dog was not sufficient 

 fully to compensate for the lowered external temperature. The 

 metabolism that is to say, the heat-production was also 

 increased. And although the food was exactly the same for 

 both animals in quantity and quality, the dog at 5 C. put on 

 less than half as much fat in the period of the experiment as 

 the ' heated ' dog, but the same amount of ' flesh.' 



The voluntary factor in the regulation of the heat-loss is of 

 great importance in man. Clothes, like hair and other natural 

 coverings, retard the loss of heat from the skin chiefly by main- 

 taining a zone of still air in contact with it, for air at rest is an 

 exceedingly bad conductor of heat. A man clothed in the 

 ordinary way has two or three concentric air-jackets around him. 

 The air in the intervals between the inner and outer garments 

 is of importance as well as that in the pores of the clothes them- 

 selves ; and it is for this reason that two thin shirts put on one 

 above the other are warmer than the same amount of material 

 in the form of a single shirt of double thickness. When a man 

 feels himself too hot and throws off his coat, he really removes 

 one of the badly conducting layers of air, and increases the rate 

 of heat-loss by radiation and conduction. At the same time 

 the water-vapour, which practically saturates the layer of air 

 next the skin, is allowed a freer access to the surface, and the 

 loss of heat by the evaporation of the sweat becomes greater. 

 The power of voluntarily influencing the heat-loss must be 

 looked upon in man as one of the most important- means by 

 which the equilibrium of temperature is maintained. In the 

 lower animals this power also exists, but to a much smaller extent. 

 A dog on a hot day puts out its tongue and stretches its limbs 

 so as to increase the surface from which heat is radiated and 

 conducted. The mere placing of a rabbit on its back, with its 

 legs apart, may cause in an hour or two a fall of i to 2 C. in 

 the rectal temperature. The power of covering themselves with 

 straw or leaves, of burrowing and of forming nests, may be 

 included among the voluntary means of regulation of the heat- 

 loss possessed by animals. A man opens the window when he 

 is too hot, and pokes the fire when he feels cold. Both actions 

 are a tribute to his status as a homoiothermal animal, and 

 illustrate the importance of the voluntary element in the 

 mechanism by which his temperature is controlled. 



The production of heat, like the loss, is to a certain extent 

 under voluntary control. Rest, and especially sleep, lessens 

 the production ; work increases it. The inhabitants of the 

 tropics, human and brute, often tide over the hottest part of 

 the day by a siesta ; and it is as natural, and as much in accord- 



