1314 PHYSIOLOGY 



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 tempera- 

 ture 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 violent exercise. Under these con- 

 ditions a man may sweat profusely when the temperature of the 

 surrounding atmosphere is at C. 



