486 THE HUMAN BODY 



out healthy life. Only in fevers, or as the result of prolonged ex- 

 posure to cold, is the balance upset. In fact its preservation is 

 necessary for the continuance of the life of a warm-blooded ani- 

 mal; should the temperature rise above certain limits chemical 

 changes, incompatible with life, occur in the tissues; for example, 

 at about 49 C. (120 F.) the muscles begin to become rigid. On 

 the other hand, death ensues if the Body be cooled down to about 

 19 C. (66 F,). 



Since we live in an environment of constantly varying tem- 

 perature a rather delicate adjustment between heat production 

 and heat loss is required. 



This adjustment is attained through the interaction of two sorts 

 of regulatory devices, one for controlling the loss of heat from the 

 Body, the other its production in the Body. As regards heat- 

 loss, by far the most important regulating organ is the skin: un- 

 der ordinary circumstances nearly 90 per cent of the total heat 

 given off from the Body in 24 hours goes by the skin (73 by ra- 

 diation and conduction, 14.5 by evaporation). This loss may be 

 controlled : 



1. By clothing; we naturally wear more in cold and less in warm 

 weather; the effect of clothes being, of course, not to warm the 

 Body but to diminish the rate at which the heat produced in it is 

 lost. 



2. Warmth through reflex vasomotor actions leads to dila- 

 tation of the skin-vessels and cold to contraction. In a warm 

 room the vessels on the surface dilate as shown by its redness, 

 while in a cold atmosphere they contract and the skin becomes 

 pale. But the more blood that flows through the skin the greater 

 will be the heat lost from the surface and vice versa. 



3. Heat induces sweating and cold checks it; the heat appears 

 to act, for the most part, reflexly through afferent cutaneous nerve- 

 fibers exciting the sweat-centers from which the secretory nerves 

 for the sudoriparous glands arise ; it may also act to some extent 

 directly on those centers, as they are thrown into activity, at least 

 in health, as soon as the temperature of the blood flowing through 

 the spinal cord is raised. In fever of course we may have a high 

 temperature with a dry non-sweating skin. The more sweat is 

 poured out, the more heat is used up in evaporating it and the 

 more the Body is cooled. 



