METABOLISM 175 



dation must take place in order to maintain the body tem- 

 perature. Hence the extent of metabolism increases with 

 lowering of the external temperature and decreases when 

 the external temperature increases. This change in meta- 

 bolism affects chiefly the combusion of fats and carbo- 

 hydrates. The increase in metabolism during loss of heat 

 is caused by a reflex increase in combustion in the muscles 

 which produces muscle contraction (shivering). 



The power of the human body to adjust itself to great 

 variations in external temperature is limited. If the external 

 temperature is very low, the loss of heat may become greater 

 than the heat production and the body temperature falls. 

 The lower the body temperature falls, the more slowly the 

 vital processes take place and the less heat is produced 

 until, at last, the processes of combustion cease altogether 

 and the organism freezes to death. If the external tempera- 

 ture is so high that the body produces more heat than it can 

 give off, the body temperature rises. This causes an in- 

 crease in the vital processes and more heat is produced until, 

 at last, the body is overheated and death results. In the 

 increased metabolism the consumption of proteids is also 

 increased. 



Within the limits in which the body can accommodate 

 itself, a fall in external temperature increases metabolism, 

 and a rise decreases it. But, outside of these limits, the 

 effects are exactly the reverse, for a decrease in external 

 temperature causes a fall, and an increase, a rise in the body 

 temperature. Hence, in the last-mentioned case, man is 

 like the cold-blooded animals, in which the metabolism rises 

 and falls with the external temperature. 



But it will be shown that, for the regulation of the body tem- 

 perature, besides the mechanism for regulating the metabolism, 

 another and far better mechanism regulating the loss of heat is 

 present (see page 181). 



(d) Effect of sensory stimulation and psychical activity. 



Stimulation of the skin and strong stimulation of the 

 retina by light increase the consumption of oxygen and 

 production of carbon dioxide. Hence during sleep the 



