METABOLISM 199 



struction of certain products of internal secretion which are 

 required in the processes of metabolism. The riddle is still 

 unsolved, but nevertheless, we are closer to its solution than 

 were our ancestors, who did not dream of the existence of these 

 vitally important compounds. 



Body Temperature. In warm blooded animals body temper- 

 ature must be maintained at a constant level, regardless of 

 external conditions. Fuel is burned in the body, and heat pro- 

 duced. In lower animals, a fall in the external temperature 

 seems directly to stimulate increased heat production in the 

 body. In man such a direct result appears to be wanting, but 

 the same end is accomplished indirectly by stimulation of shiver- 

 ing or increased voluntary activity. More fuel' is burned, and 

 more heat produced. 



It appears that the heat situation in the body is presided over 

 by a center, possibly two centers in the medulla. One of these 

 centers presides over heat production, the other over a balance 

 between heat production and heat loss. Heat loss is accom- 

 plished by increased excretion of sweat, which evaporates from 

 the skin and thus removes excess heat. Sweating may occur 

 even in cold weather during vigorous exercise, when the heat 

 production is greatly increased. If evaporation from the sur- 

 face of the body is prevented, either by coating the body with 

 some substance, or if the surrounding air is saturated with 

 moisture, great discomfort is experienced. The temperature of 

 the body rises, and death will result if the condition is not 

 relieved. 



Influence of Organs of Internal Secretion. The metabolism 

 of the body as a whole, and of specific organs or tissues is pro- 

 foundly influenced by products poured out into the blood stream 

 by certain glands and other tissues. Since these products are 

 delivered into the blood stream they are called internal secre- 

 tions. We already have considered the internal secretion of the 

 adrenals, adrenaline, in its effect upon the control of liver 

 glycogen, the internal secretion of the pancreas and its impor- 

 tance for the utilization of sugar by the muscles, the hormones 



