136 GENERAL PHYSIOLOGY 



evident from the appearance of all vital phenomena, while in the 

 latter it is for ever at a complete standstill, as is shown by the 

 absence of even the slightest phenomena of life. 



We are now in position to add a capstone to our characterisa- 

 tion of living substance — in other words, to characterise in general 

 terms the vital process itself 



It has been shown that a fundamental difference — i.e., a difference 

 in the elementary materials and the elementary forces — between 

 organisms and inorganic bodies does not exist. The vital pheno- 

 mena of organisms must, therefore, depend upon the same general 

 mechanical laws as the phenomena of the inorganic world. Bu-t 

 a difference does exist between the two great groups of bodies 

 in respect to the kind of chemical compounds in which the element- 

 ary materials are associated, since in organisms generally certain 

 highly complex compounds occur, especially proteids, which are 

 never wanting in living substance, and are never found in the 

 inorganic world. It is evident that this difference is of the same 

 kind as the differences that exist between the various inor- 

 ganic bodies themselves as regards their chemical composition. 

 Nevertheless, in the possession of the complex proteids organism.s 

 have something in common in contrast to all inorganic bodies. 



Further, it has been shown that living differ from lifeless 

 organisms, whether the latter be apparently or reallj^ dead, by their 

 metabolism — i.e., by the fact that their substance continuallj^ breaks 

 down spontaneously, is regenerated, and accordingly continually 

 gives off substances to the outside and receives other substances from 

 the outside. The kind of product arising from this decomposition 

 shows that nitrogenous compounds, especially proteids, are involved 

 in it. Since it is known that the nitrogenous proteids, with their 

 allies, which in part are derived from the proteids and in part 

 are necessary to their formation, are the sole organic compounds 

 that are never wanting in living substance, that everywhere they 

 constitute its chief mass and alone are sufficient for its formation, 

 it can be said that all living organisms are characterised by the 

 metabolism of proteids. 



We can thus summarise our considerations so far, and at the 

 same time give simple expression to the problem of all physiology. 

 The life-jprocess consists in the metabolism of proteids. If this be true, 

 all physiological research is an experiment in this field ; it con- 

 sists in following the metabolism of proteids into its details and 

 recognising the various vital phenomena as an expression of this 

 metabolism which must result from it with the same inevitable 

 necessity as the phenomena of inorganic nature result from the 

 chemical and physical changes of inorganic bodies. 



