432 SCIENCE PROGRESS. 



on receipt of a stimulus is termed its "excitability," and, 

 in proportion as this is more easy of achievement, so the 

 tissue excitability is higher. 



Muscular and nervous structures are those which show 

 in the most marked degree excitability, and they are further 

 characterised by the circumstance that the state of activity 

 when evoked, is accompanied by phenomena which are 

 readily recognised, such as movement and electrical changes. 

 In plants the leaves of Dioncea muscipulata are similarly 

 endowed with high excitability and an easily recognised 

 state of activity. It is for this reason that the term "excit- 

 able " has been more or less used as a qualification for these 

 tissues, but it is evident that the phrase "excitable tissues" 

 must on philosophical grounds have a far wider scope, and 

 may be taken to include all living structures. 



All possess a functional capacity for change, and in all 

 this change must be initiated by some physico-chemical 

 event, which is therefore the releasing agency or stimulus. 

 Even when the resultant effect is one of such slow develop- 

 ment as the division of a cell, the manufacture and discharge 

 from the cell of new substances and all the varied pheno- 

 mena which constitute growth, the functional activity thus 

 gradually displayed must be regarded as due to states of 

 excitation evoked in excitable structures by appropriate 

 although at present little known stimuli. 



The capacity for passing into new physico-chemical 

 moods and the property of being forced to perform this 

 passage in response to definite stimuli, are thus fundamental 

 characters of living as distinct from dead matter. Those 

 tissues which show the phenomena best, such as muscle and 

 nerve, enable us to grasp more completely than any others 

 upon what conditions the possession of these fundamental 

 characters depends. This is the real importance of the 

 study of the excitatory phenomena of muscle and nerve, 

 since we learn not merely their own individual traits, but 

 obtain results which serve as sign-posts, guiding us along 

 well beaten tracks towards the investigation of widely re- 

 moved living structures. 



Muscle and nerve possess in a marked degree other 



