THERMOTAXIS 667 



of 5 C. developed a thick coat of fine woolly hairs. Another dog 

 of the same litter, exposed for the same length of time to a tem- 

 perature of 31-5 to 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 maintaining 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 themselves ; 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 powei 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, lessen the pro- 

 duction ; 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 accordance with physiological 



