THE PROCESS OF EXCITATION 115 



content. When, finally, osmotic pressure within the living cell 

 and in the surrounding medium is equal, the interchange of water 

 ceases. All these processes are explained by diffusion. Self- 

 regulation takes place in this regard simply by osmotic means. 

 The conditions in respect to those decomposition products con- 

 sisting in more complicated organic combinations, such as lactic 

 acid, fatty acids and nitrogen derivatives of protein disintegration, 

 are somewhat different in that the protoplasm surface possesses 

 the property of hindering the passage of these substances into the 

 medium. These are, as is well known, first transformed by sec- 

 ondary chemical processes into transfusable substances. In this 

 transference the oxydative decomposition with the formation of 

 simpler substances plays the most important role, so that the sub- 

 stances thereby formed, namely, carbon dioxide, water and 

 ammonia, are osmotically eliminated as the result of the selective 

 permeability of the surface of the protoplasm. In this way the 

 living cell rids itself of the useless products of metabolism. 



Finally, the question remains, is the original state, as it existed 

 before the influence of the stimulus, really completely recovered 

 by metabolic self-regulation, or does even individual excitation of 

 brief duration produce a continued change in the protoplasm ? It 

 is quite impossible to prove that such an effect follows the momen- 

 tarily acting single stimulus, if stimulation has not exceeded the 

 physiological limits of intensity. Should it exist, it must be 

 imperceptible. Nevertheless, it ought to be possible by frequently 

 repeated application of the stimulus to increase this which is 

 imperceptible to an extent in which it is perceptible. This is, 

 indeed, the case and is manifested as we have already seen in 

 the increase of the volume of living substance by frequently 

 recurring functional excitation. We can, therefore, assume with 

 great probability that even the momentarily acting individual 

 stimulus produces, although not perceptible per se, lasting effect 

 in the cell. The functional excitation must be followed sec- 

 ondarily by an increase of the assimilative phase of the entire 

 cytoplastic metabolism. Otherwise the taking place of the in- 

 crease of volume of the living system following frequent excita- 

 tion of the functional constituent members of metabolism, is 



