THE TEMPERATURE OF THE BODY 



497 



cutaneous vessels, which greatly facilitates heat loss by convection 

 and radiation. Sweating is controlled by a centre in the medulla 

 which is stimulated reflexly, or directly by the temperature of the blood 

 passing through it. When this temperature is raised by warming the 

 carotid arteries, visible beads or drops of sweat are secreted by the 

 sweat glands, and these, by the cooling produced by their evaporation, 

 greatly aid in the cooling of the surface of the body. The effect 

 of raising the external temperature upon the water output, the heat 

 production, and the carbon dioxide output of the body is seen in the 

 Accompanying charts (Figs. 235, 236). 



e 8 1O 12 14- K 18 



OO 32 34 



PIG. 235. THE EFFECT OF RAISING THE EXTERNAL TEMPERATURE ox THE WATER 

 OUTPUT AND HEAT PRODUCTION. SATURATION DEFICIT INDICATES RELATIVE 

 SATURATION OF AIR WITH MOISTURE. (Rubner.) 



& 11 13 15 17 10 Zl 3d 33 XT %9 & 



FIG. 236. THE EFFECT OF RAISING EXTERNAL TEMPERATURE ON WATER AND CARBON 

 DIOXIDE OUTPUT. (Rubner.) 



The amount of heat lost in the expired air may also be regulated. 

 Warming the carotid artery leads to increased breathing, and this 

 increases the evaporation of water and heat loss. The loss of heat 

 by evaporation demands further consideration. 



The rate of evaporation from a wet surface at body temperature 

 in still air depends directly upon the vapour pressure of the surround, 

 ing atmosphere, and is independent of either the temperature or 

 barometric pressure of the atmosphere. The skin may be regarded as 

 ^ porous wall backed by capillary vessels, by means of which moisture 

 is supplied. The capillaries are profoundly affected by physical 

 conditions without e.g., constricted by cold and by physiological 

 conditions within e.g., flushed with wine. 



Simple diffusion of aqueous vapour away from the body would be 



a-2 



