86 PHYSTOLOCV CHAP. 



living organisms which chemical energy, introduced into the 

 animal body in the potential form by the different complex food- 

 stuffs, becomes free or kinetic, owing to the activity of the living 

 protoplasm. 



From this it may be deduced that the living organism is 

 distinguished from the dead in virtue of the incessant metabolism, 

 or exchange of materials, which is taking place, even when it is 

 apparently in the state of most complete repose. These material 

 exchanges are, of course, associated with exchanges of energy. 



Pfliiger, in his classical essay on Physiological Combustion in 

 Living Organisms (Pflilgers Arch. x. 1875), recognised on the 

 strength of many different experimental data that the essential 

 characteristic of living matter consists in its being highly un- 

 stable, splitting up and regenerating itself incessantly. 



" The fact," he writes, " which every biologist encounters on all 

 sides, is the amazing instability (Zersetzbarketi') of almost all living 

 matter. . . . This instability is the cause of excitability. Does 

 not the infinitesimal vital force of a ray of light evoke the most 

 potent effects in brain and retina ? I think no one will deny that 

 living matter is not merely highly unstable (zersetzbar'), but also 

 that it is continually breaking up (zersetzend}." The ultimate 

 cause of these chemical transmutations, and of the continuous 

 atomic and molecular transformations of living matter, lies in the 

 intra-uiolecular heat which conies into play in virtue of the specific 

 nature of living matter. 



In summing up his theory Pfliiger concludes : " The vital 

 process is the intra- molecular heat of the highly unstable 

 (zersetzbarer) molecules of protein present in the cell -substance, 

 which split up (zersetzender) by dissociation with formation of 

 carbonic acid, water, and starch compounds and which, on the 

 other hand, are perpetually regenerated, and also increase by 

 polymerisation." 



We saw in the last chapter that Metabolism may be regarded 

 as the result of two opposite antagonistic processes: the ana- 

 bolic, synthetic, restorative process, and the katabolic, analytic, 

 disintegrative process. E. Hering (1888) gave to the former the 

 name of Assimilation, to the latter that of Dissimilation, and laid 

 down certain important considerations in regard to the theory of 

 the intimate processes of living matter. 



He starts from the indisputable fact that living matter at any 

 given moment is the seat of two opposite processes which arise and 

 proceed simultaneously, even when no external stimulus is acting 

 on the living matter. He gave the name of autonomous assimila- 

 tion (A) and autonomous dissimilation (D) to those processes 

 which take place in living matter, when no external stimulus 

 intervenes. If these two opposite processes are equal, so that 

 neither the one nor the other predominates, then the substance 



