EXCHANGE OF MATERIAL. 469 



work has been done during the day, the elimination of carbon 

 dioxide may be decreased during the following night. In sleep 

 the metabolism of non-nitrogenized substance is indeed smaller 

 than in rest without sleep (LEVIN), and it is less the more sound 

 the sleep. 



The reason for the less abundant elimination of carbon dioxide 

 in sleep does not only depend upon muscular rest but also on 

 several other conditions, among which are the absence of light arid 

 other excitants which act in the day and which, as it seems, cause 

 a reflex of the chemical tonus of the muscles and thereby produce 

 an exchange of material. Such a regulation of the exchange of 

 material and the production of heat brought about by the nerves 

 of the skin which is produced by the influence of the chemical 

 tonus of the muscles shows that the external temperature is of the 

 greatest importance in the exchange of material. 



Action of the temperature of the surrounding air. In cold- 

 blooded animals the production of carbon dioxide increases and 

 decreases with the rise and fall of the surrounding temperature. 

 In warm-blooded animals this condition is the reverse. By the 

 investigations of LUDWIG and SANDERS-EZN, PFLUGER, DUKE 

 CHARLES THEODORE OF BAVARIA, and others, it has been demon- 

 strated that in warm-blooded animals the change in the external 

 temperature has different results, according as the animal's own 

 heat remains the same or changes. If the temperature of the 

 animal sinks, the elimination of carbon dioxide also sinks; if the 

 temperature rises, the elimination of C0 2 also rises. If, on the 

 contrary, the temperature of the body remains unchanged, then the 

 elimination of carbon dioxide increases with a lower and decreases 

 with a higher external temperature. This condition may be ex- 

 plained, according to PFLUGER and ZUNTZ, by the statement that 

 the low temperature, by exciting a reflex action in the sensitive 

 nerves of the skin, causes an increased metabolism of the muscles 

 with an increased production of heat affecting the temperature of 

 the body. The increased exchange of material produced at a low 

 external temperature only applies, as far as is known, to the non- 

 nitrogenized substances, but not to the proteids. 



Weight of Body and Age. The greater the mass of the body the 

 greater the absolute consumption of material; while, other things 



