THE TEMPERATURE OF THE BODY 1175 



clothes is partly due to the replacement of layers of air by water, which is a 

 much better conducting fluid. 



In addition to the loss of heat by convection, i.e. by warming the layers 

 of air in contact with the body, heat is also lost by radiation, i.e. by an 

 exchange of heat between the surface of the body and that of surrounding 

 cooler objects. This loss is also prevented by clothing. Since the material 

 of which clothes are made does not allow the passage of radiant heat, they 

 absorb the heat leaving the body and become warm. This warm article of 

 clothing may in its turn act as a centre for the loss of radiant heat, which 

 may again be prevented by putting on another layer. It is a familiar 

 experience that a multiplication of garments is more effective in retaining 

 the heat of the body than merely increasing the thickness of the individual 

 garments. The rate of loss of heat by radiation is diminished by a rise of 

 the amount of watery vapour in the air, since this makes the air more 

 opaque to the passage of radiant energy. Since the loss of heat depends on 

 the difference of temperature between the surface of the body and the 

 surrounding air, or objects, it will be largely affected by the temperature 

 of the skin, and therefore by the amount of blood flowing through the 

 skin. The blood-flow through the skin is under the control of the central 

 nervous system, through the vaso-motor and vaso-dilator nerves, and it is 

 by altering the size of these vessels that the central nervous system chiefly 

 acts in regulating heat loss. In cold weather, or when the heat production 

 in the body is low, the vessels are constricted, the skin is cold, and the heat 

 loss is small. On the other hand, if the temperature of the surrounding air 

 is high, or a large amount of heat is being produced in consequence of 

 muscular exercise, the vessels are dilated and the skin is hot. In hot 

 weather the dilatation in the cutaneous vessels is associated with muscular 

 inactivity, so that there is a derivation from the muscles, where the blood is 

 not required, to the skin, where a considerable circulation is necessary. 



Loss of heat by radiation and convection can only happen when the 

 temperature of the surrounding air is lower than that of the body. The 

 body temperature can, however, be maintained at its normal height in an 

 atmosphere with a temperature much higher than 37-5 C., and this in spite 

 of the fact that the production of heat in the body is still going on. In this 

 case there is a profuse secretion of sweat on the surface of the body. In the 

 evaporation of the sweat, especially if aided by a draught of air, a large 

 amount of heat becomes latent and is abstracted from the body, which is 

 therefore kept at a temperature below that of the surrounding atmosphere. 

 If the secretion of sweat is checked, by depriving an animal or man of water, 

 or if its evaporation be impeded by placing him in an atmosphere already 

 saturated with aqueous vapour, the temperature of the body runs up rapidly 

 and death ensues from hyperpyrexia, or heat-stroke. Although a man can 

 stand exposure to a temperature of 200 or even 250 F. for a considerable 

 time, provided that the air is dry, a temperature of 89 F. is rapidly fatal if 

 the air be saturated with moisture. The same mechanism comes into play 

 when the heat production in the body is very largely increased, as by 



