542 THE HUMAN BODY 



off from the Body in 24 hours goes by the skin (73 by radiation 

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

 trolled: 



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- 

 tion 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. 



Of less importance in man, but of great importance in fur- 

 bearing animals, is the loss of heat through the lungs. In warm 

 weather there is quickened respiration, brought about reflexly 

 through the play of cutaneous sensory impulses of warmth upon 

 the respiratory center. This quickened respiration carries off 

 heat more rapidly both by increasing the amount of air warmed 

 to body temperature in a given time, and by increasing the evap- 

 oration of water from the lungs. 



Our sensations induce us to add to or diminish the heat in the 

 Body according to circumstances; as by cold or warm baths, and 

 iced or hot drinks. 



As regards temperature regulation by modifying, the rate of 

 heat production in the Body, the following points may be noted; 

 on the whole, such regulation is far less important than that 

 brought about by changes in the rate of loss, since the necessary 

 vital work of the Body always necessitates the continuance of 



