CHAP. XVII.] BODILY HEAT. 197 



Distribution of heat. The blood, as we know, permeates all 

 the tissues in a system of tubes or blood-vessels. Wherever 

 oxidation takes place and heat is generated, the temperature of 

 the blood circulating in these tissues is raised. Wherever, on 

 the other hand, the blood-vessels are exposed to evaporation, 

 as in the moist membranes in the lungs, or the more or less 

 moist skin, the temperature of the blood is lowered. The gain 

 and loss of heat balance one another with great nicety, and 

 the blood, circulating rapidly, now through warmer, and again 

 through cooler tubes, is kept at a uniform temperature of about 

 100 F. In this way the whole body is warmed in somewhat 

 the same way as we warm a house, the warm blood in the blood- 

 vessels heating the tissues, as the hot water in the hot-water 

 pipes heats the rooms in steam-heated dwellings. 



Regulation of heat. We have seen that active changes in 

 the body produce heat. The action of the muscles is a source 

 of heat, the activity of the glands during digestion, the active 

 changes taking place in the tissues during inflammation or 

 suppuration, or the changes caused by some specific micro- 

 organism, and we may say that there are normal and abnormal 

 sources of heat. 



Normally, production of heat is balanced by loss of heat, and 

 the chief regulator of this gain and loss is undoubtedly the 

 skin. This is well seen in the case of muscular exercise. 

 Every muscular contraction gives rise to heat, and yet during 

 severe muscular exercise the temperature of the body does not 

 rise, or rises only to a trifling extent. This is accounted for 

 by the fact that when the muscular exertion causes the blood 

 to circulate more quickly than usual, the blood-vessels in the 

 skin dilate, the sweat-glands at the same time are excited to 

 pour out a more abundant secretion, and the heated blood pass- 

 ing in larger quantities through the cutaneous vessels (which 

 are kept well cooled by the evaporation of the perspiration), 

 the general average temperature of the body is maintained. 



In pyrexia, or fever, rise of temperature is due to some cause 

 which, while increasing the metabolism of the tissues, at the 

 same time interferes with the process by means of which the 

 body rids itself of superfluous heat. We all know how hot and 

 dry the skin is liable to become in fevers ; how we try to restore 

 its function and lower the temperature by baths, sponging, and 



