480 GENERAL PHYSIOLOGY 



containing salts taken from the earth. Regarding fat, it is known 

 that it can serve for the construction of carbohydrate by 

 transformations in the plant ; the carbohydrate then gives off in 

 turn the material for the formation of proteid, for in the seeds of 

 PcBonia, which are filled with fatty oils, all oil disappears, e.g., 

 after long exposure to the air, and starch appears in its place. It 

 is thus seen most clearly in the plant how different substances 

 serve for the construction of the proteid molecule; but the animal 

 demonstrates best the fact that the most important non-nitro- 

 genous groups of atoms in living substance, especially carbohydrates 

 and fats, can be derived from the decomposition of the proteid 

 molecule.^ Thus, the fact that fat can be derived from proteid has 

 been demonstrated by Leo in his experiments on phosphorus 

 poisoning in frogs, and by Franz Hofmann in his experiments on 

 the nutrition of the larvse of flies with blood freed from fat. 

 Further, Claude Bernard and recently Mering have proved upon 

 dogs whose bodies were freed from glycogen by fasting, that after the 

 feeding of proteid glycogen is again manufactured in great quantity, 

 in other words, that this carbohydrate can be derived from the 

 transformation of proteid. Finally, Gaglio has established the fact 

 that the lactic acid in the body is derived from the transformation 

 of the proteid molecule, since the quantity of it in the blood is 

 dependent solely upon the quantity of proteid that is eaten. 

 Regarding the nitrogenous excretory products of the body, it is 

 evident that they can be derived only from the transformation of 

 proteids and their compounds, since no other nitrogenous bodies 

 are present among the essential organic compounds of living sub- 

 stance. But the most striking proof of the fact that all substances, 

 both non-nitrogenous and nitrogenous, that are essential to the 

 life of the cell, can be derived hy chemical transformation from 

 proteids, is afforded by one of the most significant facts of physio- 

 logy, namely, the possibility that carnivora are capable of main- 

 taining their life upon pure proteid and, as Pfliiger ('91) has recently 

 shown, possess great capacity for doing work. Nothing demonstrates 

 better than this fact the controlling position of the proteid 

 molecule in the vital process. 



Hence, not only does it follow from the fact of metabolism that 

 very labile complexes of atoms exist in living substance, with the 

 presence of which life is inseparably associated, but it is the pro- 

 teids whose presence constitutes the general, essential condition 

 and focus of life. If we endeavour to harmonize these two facts, the 

 unavoidable necessity arises of assuming in living cell-substance, be- 

 sides the known proteids that occur also in dead substance, certain 

 other proteids or compounds of proteids, that are present in life 

 only and terminate life with their decomposition. 



Dead proteid, as it is found in the dead egg of the fowl, or as it 



' Cf. p. 163. 



