1876.] 



Excitability of Motor Nerves. 



9 



of the frog ; but the results yielded by this method were not more uni- 

 form than those which I had previously obtained by the method of 

 rapidly preparing and observing excised muscles. 



§ 2. If the gastrocnemius of a frog be placed on non-polarizable elec- 

 trodes in the position already described in § 1, and if care has been taken 

 not to injure the attached sciatic nerve, I find that upon now dividing 

 tliis nerve, either near or just within the muscle, remarkable alterations 

 ensue, not only, as is already known, in the general sensitiveness of the 

 muscle, but also, and more particularly, in its relative sensitiveness to 

 make and to break of the current. The following are the mean results 

 yielded by a large number of experiments : — 



Descending 



Ascending 



Descending 



Ascending 



make 



make 



break 



break 



before 



after 



before 



after 



before 



after 



before 



after 



cutting. 



cutting. 



cutting. 



cutting. 



cutting. 



cutting. 



cutting. 



cutting. 



24 



27 



36 



46 



2 



32 



1 



H 



In this Table the word " descending " means passage of the current 

 from the femoral to the tarsal end of the gastrocnemius, and "ascending," 

 of course, passage of the current in the opposite direction. " Cutting " 

 means section of the sciatic nerve just after it enters the muscle ; and the 

 numbers represent the relative sensitiveness of the muscle to the stimuli 

 which are indicated above them*. I have appended a diagram (p. 11), 



* The numbers are thus obtained : — Suppose A to be the battery, B a set of resist- 

 ance-coils, C a rheochord, D a commutator, and E the muscle. By removing a plug 



from B the resistance is mcreased, and therefore the current through E is diminished. 

 But the effect of remoTing a plug from C, although likewise that of increasing the 

 resistance through the whole circuit., is to augment the current passing through E. 

 For, previous to removing a plug from C, the current branched at x, and the resistance 



