V TEMPERATURE OF THE BODY 205 



And it appears that when the body is exposed to a high 

 temperature these nerves are so affected as to lead to a 

 dilation of small arteries of the skin ; but when these 

 are dilated the capillaries and small veins in which they 

 end become mucli fuller of blood, and from tliese filled 

 and swollen capillaries much more nutritive matter passes 

 through the capillary walls to the sweat-glands, so that 

 these have more abundant material from which to manu- 

 facture sweat. On the other hand, when the body is 

 lowered in temperature the vaso-motor nerves are so 

 affected that the small arteries of the skin are constricted ; 

 hence less blood enters the capillaries of the skin, and 

 less material is brought to the sweat-glands. 



Thus when the temperature is raised two things happen, 

 both brought about by the nervous system. In the first 

 place, the arteries of the skin are widened so that a nmch 

 larger proportion of the total blood of the body is carried 

 to the surface of the skin and there becomes cooled : and, 

 secondly, this cooling process is greatly helped by the in- 

 creased evaporation resulting from the increased action of 

 the sweat-glands, whose activity is further favoured by the 

 presence in the skin of so much blood. Conversely when 

 the temperature is lowered, less of the blood is brought to 

 the skin, and more of the blood circulates through the 

 deeper, hotter parts of the body, and the sweat-glands 

 cease their work (this quiescence of theirs being in turn 

 favoured by the lessened blood-supply) ; hence the evapo- 

 ration is largely diminished, and thus the blood is much 

 less cooled. 



Hence it is that, so long as the surface of the body per- 

 spires freely, and the air passages are abundantly moist, a 

 man may remain with impunity, for a considerable time, 

 in an oven in which meat is being cooked. The heat of 

 the air is expended in converting this superabundant per- 

 spiration into vapour, and the temperature of the man's 

 blood is hardly raised. 



13. Regulation of Body-temperature by Altered 



