114 IRRITABILITY 



ture is concerned, it is self-evident that this likewise does not 

 constitute an essential difference, for we are acquainted with con- 

 ditions of equilibrium in chemical reactions possessing a number 

 of members and in inhomogeneous mixtures. Finally, the fact 

 that the reaction in the living system is not totally reversible, 

 forms no barrier to the assumption in principle of metabolic self- 

 regulation as a chemical equilibrium. It is quite possible to con- 

 ceive of a chemical equilibrium in a reacting mixture, of which 

 only certain constituent processes are reversible, without the 

 totality of the reactions as a whole being necessarily so. Let us 

 assume, by way of example, that the assimilative processes of 

 the metabolic chain are reversible, then under constant quantita- 

 tive relations of foodstuffs, following every disintegration of 

 assimilative products with removal of the decomposition products, 

 the same amount of assimilatory processes is required for build- 

 ing up. And this is just that which we observe in metabolic 

 equilibrium. Accordingly, we may look upon the metabolic equi- 

 librium as a special, although a very highly complicated, instance 

 of chemical equilibrium, and we may explain the metabolic self- 

 regulation following a dissimilative excitation of the same, by 

 those principles on which the rebuilding of chemical equilibrium 

 is founded. It is true that the special details of this process can 

 be differentiated in only that degree in which it is possible to 

 penetrate at all into the details of metabolism of the given cell 

 form. In this, as is well known, the advance is extremely slow. 



The rebuilding process following decomposition of living sub- 

 stance in response to an excitating stimulus consists not merely 

 in compensation for the decomposed atom groups but also in the 

 removal of disintegration products. This removal can be accom- 

 plished, in so far as simple chemical substances such as carbon 

 dioxide and water are concerned, by diffusion. Observations 

 have shown that the semi-permeable protoplasm surface is per- 

 vious to water and carbon dioxide. The latter can, therefore, 

 depending upon the amount of concentration, be eliminated from 

 the living substance. Output of water likewise takes place in so 

 far as the specific water content of the living substance is ex- 

 ceeded and which is osmotically regulated by its amount of salt 



