LECTURE XXXIV. 



GENERAL CONSIDERATIONS ON IRRITABILITY. 



With the word irritability I designate the mode of reaction to stimuH, 

 which is peculiar to living organisms. This is of course in the first place a 

 mere arbitrary definition of a word; but such definitions of terms are necessary 

 for mutual intelligibility, although they are unfortunately often neglected in the 

 province of natural science. That such a bare definition of the term — which 

 moreover I have come to regard as right only after many years of thought — has its 

 uses, is illustrated directly by the following remarks. I say the mode in which living 

 organisms only react to stimuli, is irritability : by this I imply at the same time that all 

 irritability in the tissues is due to the protoplasm. For there is no difference of 

 opinion as to this one point, that all the processes of life depend upon proto- 

 plasm, and that where no protoplasm is present no vital processes can occur. It 

 follows thence, however, that the essential cause of all phenomena of irritability 

 must be situated in the protoplasm, because we regard irritability as a property of 

 living organisms only. And as a matter of fact, in all cases where the phenomena of 

 irritability have been successfully and sufficiently analysed, the investigations have led 

 to the result that living protoplasm plays a chief part in the matter; though this 

 is not intended to imply that a phenomenon of irritability is conditioned entirely and 

 solely by the properties of protoplasm. On the contrary, mechanisms which are 

 influenced in a secondary manner only by the stimulated protoplasm determine to a 

 large extent the final external character of a phenomenon of irritability, especially in 

 a complex organ. At the same time it follows from what has been said, that all 

 living cells, cell-tissues, and living organs of the plant must be irritable, according to 

 our definition ; for they all contain protoplasm, and there is no doubt that all proto- 

 plasm is irritable, at least in certain conditions — i. e. it reacts to external stimuli in a 

 way which lifeless bodies do not as a rule do ; although, as we shall see later, reactions 

 occur even in crystals which present a very striking similarity to certain phenomena 

 of irritability. 



On the other hand, our definition of irritability excludes the assumption that 

 every reaction of organised bodies whatever may be regarded as irritability. For, in 

 the first place, organised bodies, quite apart from their specific peculiarities, are also 

 at the same time physical objects, and must react to external influences according to 



