34 IRRITABILITY 



the amoebae under the vital conditions existing in tap water have 

 Umax form. The vital conditions undergo a change by the addi- 

 tion of a solution of caustic potash, which acts as a stimulus. The 

 consequence is a reaction, in which the animal assumes radiosa 

 form. By the action of the stimulus a new state of the living 

 substance is produced, and remains as long as the solution of 

 caustic potash is contained in the medium. The solution of 

 caustic potash is, therefore, a stimulus for the state of the vital 

 system, which is manifested in the Umax form, whilst for the 

 state of the system which shows itself in the radiosa form, it is a 

 vital condition. If I place the amoebae of the radiosa form once 

 again in tap water, they assume the proteus and then the Umax 

 form. The withdrawal of the solution of caustic potash, the pres- 

 ence of which is a vital condition for the radiosa state, acts as a 

 stimulus, which results in a transition of the vital system to 

 another state. By altering the medium I can at will bring about 

 this change of form in the same individuals. In this way one and 

 the same factor can figure as stimulus and vital condition, accord- 

 ing to the state of the vital system on which it acts. Whilst its 

 addition acts as stimulus in the one state, its withdrawal acts as a 

 stimulus in the other state, which it has produced. The same 

 fact is shown by the well-known example of Artemia salina, which 

 on being placed in fresh water changes into Branchipiis stagnalis 

 and, when again introduced into sea water, becomes once more 

 Artemia salina. 



These facts show clearly that some stimuli can also be con- 

 sidered as vital conditions. In the absence of certain stimuli, 

 life could not exist for any length of time. Irt the highly dif- 

 ferentiated cell community of the animal organism, for instance, 

 as a result of the coexistence of the cells and the tissues, many 

 parts have forfeited in a measure their independence. An 

 example of this is the skeletal muscle, which, in the absence of 

 impulses from the nervous system, reaches a low level of chemi- 

 cal change and energ)^ transformation. Here the nervous impulses 

 which act as momentary stimuli, are also in the course of time 

 indispensable vital conditions. Without them the muscle would 

 gradually become atrophied from inactivity. The same applies 



