METABOLISM 267 



when standinsr at ease, even although in the former case 

 there is no visible movement. The lowest rate of meta- 

 bolism is reached in sleep when the skeletal muscles are 

 relaxed and the action of the other muscles is reduced to a 

 minimum, 



2. Rate of Cooling. 



Since under ordinary conditions the temperature of the 

 surrounding air is lower than that of the body, there must 

 be a constant loss of heat and consequently a constant 

 production of heat to keep the temperature of the body 

 steady. 



Loss of Heat. 



1. Skin. — Heat is lost from the body by the skin in 

 two ways : — (a) by conduction and radiation, and (b) by 

 evaporation of sweat. 



(rt) Conduction and Radiation. — The extent of this loss 

 depends upon the difference between the temperature of the 

 body and that of the air. Radiation plays the most important 

 part when an animal is at rest in still air ; conduction when 

 the exchange of air over the surface is rapid, as in wind. 



The influence of variation in the temperature of the air 

 is minimised by the covering of fur or feathers, which retains 

 a stationary layer of air of about 25° to 30° C. over the 

 skin. 



Cold stimulates the growth of hair. On the other hand, 

 the heavy winter coat tends to be shed when an animal is 

 confined in a warm atmosphere. 



(6) Evaporation of Siueat — Heat is rendered latent by 

 the evaporation of sweat, and is taken from the body which 

 is thus cooled, just as the hand may be cooled by allowing 

 ether to evaporate upon it. The extent of loss depends not 

 only on the amount of sweat secreted, but also upon the 

 rapidity with which the evaporation goes on. This is governed 

 by the dryness and temperature of the air, and the rapidity 

 of its exchange by wind and other air currents. 



The horse sweats easily. Working under ordinary con- 

 ditions of climate, it may lose from 5 to 10 litres of water 

 as sweat per day. 



