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to the leg, is direct derived current. The diagram will also seem to 

 show that the thigh ought to contract at the end of the current, for 

 the thigh is traversed by inverse derived current. In fact, however, 

 the thigh does not contract either at the beginning or at the end of 

 the current ; and this perhaps is not to be wondered at ; for the 

 author finds that contraction attends upon the beginning of a direct 

 current of a given strength for some time after it has ceased to 

 attend upon the end of an inverse current of the same strength. 



3. The modus operandi of galvanism upon a motor or mixed nerve 

 is a subject beset with difficulties ; but some of these difficulties do 

 not appear to be altogether insurmountable. 



(a) In looking at the movements belonging to the first period 

 that of double contraction (vide Table I.) it is not difficult to find 

 a reason which will in some degree explain how it is that contraction 

 is confined to the beginning and end of the current. It is not diffi- 

 cult to see that the beginning and ending of the galvanic current in 

 the nerve may involve certain changes in the strength of the nerve- 

 current, and that these changes may in their turn give rise to 

 momentary induced currents in the nerve and in the neighbourhood ; 

 for such momentary currents are induced, not only when a current 

 begins to pass and when it ceases to pass, but also at the moments 

 when it undergoes any change of strength. It is not difficult to see, 

 also, that the muscular fibres to which the nerve is distributed may 

 be the seat of some of the secondary currents thus induced, and that 

 these fibres may be thrown into contraction by these currents. Nor 

 is it difficult to see if the contraction be thus connected with the 

 induced currents that there will be no contraction in the interval 

 between the beginning and ending of the inducing galvanic current ; 

 for if the inducing current exhibits no variation in strength, there is 

 no secondary current induced in this interval. 



(b) It is, perhaps, too much to expect at present a full explana- 

 tion of the second period of contraction of that period, that is, in 

 which the contraction alternates at the beginning of the direct and at 

 the end of the inverse current ; but the author is disposed to think 

 that a partial answer may be found in the collation of the three facts 

 which follow. 



The first fact is this that the direction of the nerve-current in the 

 sciatic and lumbar nerves of a frog (except in those last moments of 



