SOME RECENT WORK UPON MUSCLE AND NERVE. 435 



cited of conditions which change excitability in one direction 

 and functional capacity in the other ; and since there are 

 also instances in which the two both vary the same way, it 

 is clear that the molecular foundations of excitability and 

 functional capacity are not so firmly linked together that a 

 change in the one must produce a corresponding change in 

 the other. 



We may picture to ourselves the living metabolism in any 

 one region of a tissue, such as nerve, as consisting on the 

 one hand of the tightening, and on the other of the loosen- 

 ing of complex molecular bonds. An apt illustration is 

 afforded by the stability of a house built of cards, and having 

 such a roof, that it is in itself an unstable structure. A 

 slight shake of the table (the stimulus) can secure the 

 downfall of such a house, and this downfall we may liken 

 to the state of excitation. 



Its degree of instability we may term its excitability ; the 

 more this is increasd, the more readily is the fall evoked. 

 On the other hand the magnitude of the fall depends upon 

 the number of cards which constitute the house, and as these 

 are more numerous the capacity of the house to produce a 

 large downfall is obviously increased. 



Functional capacity is measured by the magnitude of 

 the state of excitation, hence in the illustration functional 

 capacity will be increased in direct proportion to the number 

 of cards constituting the house and capable of being over- 

 thrown in its fall. 



Now it is possible to make the house more stable by 

 buttressing its sides with cards ; it is now less readily 

 upset (less excitable) ; on the other hand, if it should 

 be upset the number of cards involved in its fall is 

 increased (increased functional capacity). But it is also 

 possible to render it more stable by removing those top- 

 heavy cards which give to the roof its inordinate size and 

 weight. Under these circumstances it is more stable (less 

 excitable), but there are fewer cards to fall when it is up- 

 set (diminished functional capacity). The illustration will 

 not bear being pushed, unless we introduce a human agency 

 for the process of restoration, the rebuilding of the house 



