SOME RECENT WORK UPON MUSCLE AND NERVE. 431 



vital activity as displayed by those methods of inquiry now 

 utilised by physiologists. 



Living tissues are characterised by possessing peculiar 

 properties, one of which is that of undergoing- changes, the 

 real causation of which is unknown to us. The changes are, 

 however, readily recognised since they display themselves 

 as physico-chemical effects of a definite and distinctive 

 character. Each tissue shows a group of such effects which 

 becomes more special as the tissue itself becomes differenti- 

 ated in structure. The display of such definite changes is 

 the sign to the physiologist that the tissue possesses vitality; 

 the potentiality for such display may be termed its capability 

 of functional activity, or, in short, "functional capacity". 



This functional capacity is swayed by such conditions as 

 the chemical nature of the environment, etc., it is thus not 

 a constant quantity but one which varies, now increasing, 

 now diminishing. 



Its complete abolition beyond all power of recovery 

 constitutes death, and thus the essence of the mysterious 

 and unknown quantity termed life, is the possession by the 

 tissue of this capacity for undergoing change of a definite 

 character. It is, therefore, the possibility of the appearance 

 of a new set of physico-chemical phenomena in such tissues 

 as muscle and nerve which constitutes the fundamental 

 characteristic of the living as distinct from the dead state. 

 In these tissues the condition of activity can be evoked 

 by producing physico-chemical changes in their substance. 

 The evoking change acts in such cases, not as the causative 

 factor of the subsequent active phenomena, but as an agent 

 which releases, and thus initiates, a whole series of more 

 subtle unknown chemical disturbances, the gross result of 

 which we recognise as distinctive. Its action is analogous 

 to that of the electrical current which fires a submarine 

 mine and thus initiates an explosion. The releasing or 

 initiating agency, in the case of the tissue, is termed the 

 exciting cause or the stimulus, whilst the subsequent 

 effects evoked by this are defined as those of the " state 

 of excitation ". Finally, the possession by any tissue 

 of susceptibility to be brought into the state of excitation 



