484 THE HUMAN BODY. 



N. 



however, be explained in another way, the rise being at- 

 tributed to a sudden diminution of loss from more exposed 

 parts of the skin, dependent on contraction of the cutaneous 

 arteries. In some cases, however, the temporary rise is accom- 

 panied by an increased excretion of carbon dioxide, which 

 would indicate that the surface cooling does really increase 

 the oxidations of the Body. 



5. Certain drugs, as salicylic acid, and perhaps quinine, 

 diminish the heat production of the Body. Their mode of 

 action is still obscure. 



On the whole, however, the direct heat-regulating mech 

 anisms of the Human Body itself are not very efficient, 

 especially as protections against excessive cooling. Man 

 needs to supplement them by the use of clothing, fuel, and 

 exercise. 



Local Temperatures. Although, by the means above 

 described, a wonderfully uniform bodily temperature is 

 maintained, and by the circulating blood all parts are kept 

 at nearly the same warmth, variations in both respects do 

 occur. The arrangements for equalization are not by any 

 means fully efficient. External parts, as the skin, the lungs 

 (which are really external in the sense of being in contact 

 with the air), the mouth, and the nose chambers, are always 

 cooler than internal; and even all parts of the skin have not 

 the same temperature, such hollows as the armpit being 

 warmer than more exposed regions. On the other hand, a 

 secreting gland or a working muscle becomes warmer, for 

 the time, than the rest of the Body, because more heat is 

 liberated in it than is carried off by the blood flowing 

 through. In such organs the venous blood leaving is warmer 

 than the arterial coming to them; while the reverse is the 

 case with parts, like the skin, in which the blood is cooled. 

 An organ colder than the blood is of course warmed by an 

 increase in its circulation, as seen in the local rise of temper- 

 ature in the skin of the face in blushing. 



Thermogenic Nerves. All nerves, such as motor or 

 secretory, which can throw working tissues into activity are 

 in a certain sense thermogenic nerves, since they excite in- 

 creased oxidation and heat production in the parts under 

 their control. A true, purely thermogenic nerve would be 

 one which increased the heat production in a tissue without 

 otherwise throwing it into activity; and whether such exist 



