36 IRRITABILITY 



death or, on the other hand — and this, as Weissman especially 

 emphasizes, is realized in the case of unicellular organisms — a 

 corrective process takes place, a process of cell division by which 

 the original state of the cell is restored and development begins 

 anew and in a similar manner. 



Ought we to designate these constant alterations in the inner 

 vital conditions as "stimuli"? Usage in this connection has 

 already answered in the negative, by applying to them the word 

 "development." And this use is in a certain sense justified. 

 Let us imagine an organism or any other object for the purpose 

 of investigation as isolated from its surroundings. This con- 

 ception, which we have already stated, proves untenable on closer 

 analysis, but it, however, is based on the nature of the meth- 

 ods of human observation and is indispensable for practical use 

 within certain limits. Then the inner vital conditions belong to 

 the organism, the external to the medium. They differ in so far 

 that the external vital conditions can exist permanently without 

 alteration, that is, independently of the development of living 

 systems, whilst the inner vital conditions of every living organism 

 continuously and progressively undergo alteration. In this sense, 

 but only in this, there is evidently a difference between the inner 

 and outer vital conditions, which permits a separation of the two 

 groups. But we should always bear in mind that this separation 

 cannot be sharply defined. On the same basis we assume that 

 the organism for purposes of study is separated from its sur- 

 roundings as an independent system, which leads us in conse- 

 quence to contrast the alterations in the internal with those in 

 the external vital conditions, in which we designate the first as 

 processes of development, the latter as stimuli. This distinction, 

 as all differentiations and separations in nature, gives us only a 

 practical working basis. 



In this way we confine the conception of the stimulus to all 

 alterations in the external vital conditions of a living system, 

 considered as isolated. This view does not exclude the fact 

 that stimuli can also occur and act within an organism. If a ner- 

 vous impulse is conducted from the cerebral cortex through the 

 pyramidal tract to a skeletal muscle, this impulse acts upon the 



