5 o8 A MANUAL OF PHYSIOLOGY 



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 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 in- 

 cluded 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 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 



