METABOLISM 225 



but most recent evidence indicates that cod liver oil contains 

 some substance other than A, the taking of which makes it pos- 

 sible for an animal to make better use of the calcium in his 

 food than without this substance. This substance, D, is cura- 

 tive for rickets. But rickets also may be relieved or cured by 

 giving phosphates, or exposure to sunlight. The case of rickets 

 is thus by no means clear. 



Other disorders have been reported as deficiency diseases, but 

 the evidence is not conclusive. Doubtless time will bring to 

 light other vitamins, and solve many of the problems in this 

 field which still are unsettled. 



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 qf 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 



