914 PHYSIOLOGY 



mother-substances of hormones and other chemical compounds which 

 play a dynamic rather than static part in the phenomena of life, and 

 supply conditions of activity rather than material for the production 

 of energy. The carbohydrates not only act as sources of energy, but 

 are necessary to the building up of the proteins into the protoplasmic 

 complex. Without them moreover this complex cannot properly 

 utilise the fat contained in itself or supplied in its food. On the 

 other hand, the carbohydrates by themselves are not available as food, 

 but require some connecting link, which may be protein or nitrogenous 

 in character, to enable their association with the active part of the 

 protoplasm and their utilisation by oxidation. At the same time 

 there is a certain possibility of interconversion between these different 

 substances ; sugar may be formed from proteins, fats from carbo- 

 hydrates. On the other hand, the formation of fats from proteins is 

 apparently impossible in the cells of the higher animals, and the 

 evidence for the formation of sugar from fat is limited to the study of 

 the respiratory quotient in hibernating animals. With the exception 

 of a few cases quoted by Pfliiger and von Noorden, no support for 

 such a conversion is obtained from the conditions observed in the 

 glycosuria caused by the administration of phloridzin or by extirpation 

 of the pancreas. 



