490 GENERAL PHYSIOLOGY 



or depress the process of dissimilation, we shall term dissimilatory 

 stimuli. 



Upon the other side, it is clear that assimilation also can be pro- 

 moted by external influences. This process depends upon the union 

 of chemical affinities belonging to both the residue of the biogens and 

 the complete biogen molecules themselves, the participation of the 

 latter following from their inclination to polymerisation. Hence 

 all those factors can increase assimilation, which procure and put 

 into proper form the substances that are necessary for the union of 

 the existing affinities. The increased introduction of food-material 

 and oxygen is more than all else efficient in this direction. Exam- 

 ples of other agencies are light in the cells of green plants, which is 

 necessary to split up carbonic acid and make carbon available, 

 and all stimuli that incite the production of ferments, which are 

 needed to make solid food-stuffs soluble. But, on the other 

 hand, there are factors that depress the process of assimilation. 

 Special examples of these are lack of food and oxygen, in the plant- 

 cell lack of light, and the absence of ferments. We shall term 

 all these factors that either excite or depress the process of assimi- 

 lation, assimilatory stimuli. 



Four important cases of reactions can thus be distinguished 

 Stimuli are able to produce : 



1. Excitation of dissimilation. 



2. Depression of dissimilation. 



3. Excitation of assimilation. 



4. Depression of assimilation. 



But the possibilities are not yet exhausted. For the individual 

 events in living substance are in extremely close correlation with 

 one another, and, as has been seen, in certain cases a complete 

 internal self-regulation of metabolism is thereby occasioned, so 

 that, e.g., every change of assimilation results in an equal change of 

 dissimilation. Hence it is possible that a stimulus can call forth 

 simultaneously excitation or depression of both dissimilation and 

 assimilation. The following must, therefore, be added to the four 

 cases above : 



5. Total excitation. 



6. Total depression. 



In connection with these it is to be noticed that different parts 

 of the metabolic series can be excited or depressed in unequal de- 

 grees. 



But still other possibilities are conceivable. Internal self- 

 regulation of metabolism does not exist everywhere, and where it 

 exists it is confined within certain limits ; for, if it were effective at 

 all times and in all places, continual metabolic equilibrium would 

 exist, and growth, development, and atrophy would be impossible. 

 Hence, cases are conceivable in which a stimulus produces simul- 

 taneously excitation of assimilation and depression of dissimilation, 



