INTR OD UCTION. 1 9 



external stimuli, but as yet it has not been possible to show that the forma- 

 tion or conduction of a nerve impulse is accompanied by or dependent upon 

 the liberation of potential chemical energy. The nature of the reaction of 

 irritable living matter is found to vary with the character of the tissue or 

 organism on the one hand, and, so far as intensity goes at least, with the 

 nature of the stimulus on the other. Response of a definite character to 

 appropriate external stimulation may be ojbserved frequently enough in 

 dead matter, and in some cases the nature of the reaction simulates closely 

 some of those displayed by living things. For instance, a dead catgut string 

 may be made to shorten after the manner of a muscular contraction by the 

 appropriate application of heat, or a mass of gunpowder may be exploded by 

 the action of a small spark and give rise to a great liberation of energy which 

 had previously existed in potential form within its molecules. As regards 

 any piece of matter we can only say that it exhibits vital irritability when the 

 reaction or response it gives upon stimulation is one characteristic of living 

 matter in general or of the particular kind of living matter under observation ; 

 thus, a muscle-fibre contracts, a nerve-fibre conducts, a gland-cell secretes, an 

 entire organism moves or in some way adjusts itself more perfectly to its 

 environment. Considered from this standpoint, irritability means only the 

 exhibition of one or more of the peculiar properties of living matter and can- 

 not be used to designate a property in itself distinctive of living structure ; 

 the term, in fact, comprises nothing more specific or characteristic than is 

 implied in the more general phrase vitality. When an amoeba dies it is no 

 longer irritable, that is, its substance no longer assimilates when stimulated by 

 the presence of appropriate food, its conductivity and contractility disappear 

 so that mechanical irritation no longer causes the protrusion or retraction of 

 pseudopodia no form of stimulation, in fact, is capable of calling forth any 

 of the recognized properties of living matter. To ascertain, therefore, whether 

 or not a given piece of matter possesses vital irritability we must first become 

 acquainted with the fundamental properties of living matter in order to recog- 

 nize the response, if any, to the form of stimulation used. 



Nutrition or assimilation, in a wide sense of the word, has already been 

 referred to as probably the most universal and characteristic of these prop- 

 erties. By this term we designate that series of changes through which dead 

 matter is received into the structure of living substance. The term in its 

 broadest sense may be used to cover the subsidiary processes of digestion, 

 respiration, absorption, and excretion through which food material and 

 oxygen are prepared for the activity of the living molecules, and the waste 

 products of activity are removed from the organism, as well as the actual 

 conversion of dead material into living protoplasm. This last act, which is 

 presumably a synthetic process effected under the influence of living matter, 

 is especially designated as anabolism or as assimilation in a narrower sense 

 of the word as opposed to disassimilation. By disassimilation or katabolism 

 we mean those changes leading to the destruction of the complex substance of 

 the living molecules, or of the food material in contact with these molecules. 



