LA W OF EXCITATION. 



469 



same series of stimuli. The character of the two sets of results is 

 shown in the following table : — 



It will be noticed that the nerve change increases in proportion as 

 the intensity of the stimulus is augmented, and that this increase still 

 occurs when the muscular response, having reached its maximum limit, 

 is unable to indicate the augmentation. The relationship of electrical 

 response to nerve stimulus will be again referred to in the section 

 dealing with the electromotive phenomena. 



2. The time relations of the change (the law of excitation). — 

 These have been especially studied in connection with electrical modes 

 of stimulation, and their consideration may be appropriately prefaced 

 by a statement of the results arrived at by du Bois-Eeymond, the 

 consideration of which caused him to formulate the two propositions 

 which constitute his " law of excitation." 1 



(a) It is not the absolute intensity of a current at any given moment 

 which constitutes its effectiveness for the excitation of a motor nerve, 

 but the variation in such intensity from one moment to another. 



(b) The excitation by current variation with any given quantity is 

 the more pronounced the more rapidly such variation is effected, i.e., 

 the greater the amount of such variation in a unit of time. In other 

 words, the potency of an electrical current for the excitation of an 

 excitable tissue such as nerve, is not so much a function of I as of 

 dl 



jm where I represents the intensity and T the time. 



There are here two statements, — the first indicates that it is the 

 increase or decrease, the onset or cessation, of the flow of a current, and 

 not its maintenance, which determines its exciting efficiency ; the second, 

 that in proportion as the onset or cessation is more rapidly effected, the 

 exciting efficiency is increased. 



The foundation for the first statement is a series of well-known and 

 uncontested facts ; they may all be sufficiently illustrated by one 

 example, that of the exciting value of a galvanic current, when this is led 

 through the sciatic nerve of the frog. The muscle to which such nerve 

 is attached, responds by a single twitch synchronous with the closure, 

 and by a second twitch synchronous with the opening, but during the 

 maintenance of the closure it remains quiescent. 



Although this and other facts of a similar nature appear to prove 

 the validity of the first part of du Bois-lieymond's law, there are many 

 considerations which forbid its acceptance as a general law of 

 excitation. 



As already indicated, the muscular response, when present, is an 

 1 E. du Bois-Reymond, '• Untcrsuchungen," 1849, Bd. i. 



