204 ELEMENTARY PHYSIOLOGY less. 



which should be able to evaporate freely — as, for example, 

 by wrapping them in wet cloths. And the greater the 

 quantity of water thus evaporated, the lower would be 

 the temperature of the whole apparatus. 



Now, the regulation of the tenipei'atui-e of the human 

 body is chiefly effected on this principle. The vessels are 

 closed pipes, but a great number of them are inclosed in 

 the skin and in the mucous membrane of the air-passages, 

 which are, in a physical sense, wet cloths freely exposed to 

 the air. It is the evaporation from these which exercises 

 a more important influence than any otiier condition upon 

 the regulation of the temperature of the blood, and, con- 

 sequently, of the body. 



But, as a further nicety of adjustment, the wetness of 

 the regulator is itself determined, through the aid of the 

 nervous system, by the temperature of the body. The 

 sweat-glands, as we have seen,, may be made to secrete by 

 impulses reaching them along certain nerves coming from 

 a centre in the central nervous system. This centre is 

 itself connected by other nerves with the skin, and the 

 ends of these cutaneous nerves are so constituted that 

 they are stimulated by heat applied to the skin. When 

 the body is exposed to a high temperature (and the same 

 occurs when a part only of the body is heated), these 

 cutaneous nerves convey impulses to the central nervous 

 system, from which other impulses are then sent out 

 along the secretory nerves to the sweat-glands and cause 

 them to pour forth a copious secretion on to the skin ; 

 and when the temperature falls, the glands cease to act. 

 Moreover, in this work of secreting sweat, the sweat- 

 glands are assisted by corresponding changes in the blood- 

 vessels of the skin. It has been stated (see p. 68) that 

 the small arteries of the body may be sometimes narrowed 

 or constricted, and sometimes widened or dilated. Now 

 the condition of the small arteries, whether they are 

 constricted or dilated, depends, as we have also seen, 

 upon the action of certain nerves (vaso-motor nerves). 



