ACTION OF NERVES ON MUSCLES. 229 



the continuance of its electrotonic state, but only at its commencement 

 and end; and it appears to result from still more intimate observa- 

 tions, made by competent experimenters, that contraction takes place 

 at the moment of occurrence of the catheleutrotonic state, but at the 

 cessation of the anelectrotonic state. The power of producing these 

 contractions in a muscle, is therefore dependent, not only on the 

 strength of the current, but is modified by its particular direction, 

 upwards or downwards, through the nerve. The change of state pro- 

 duced by irritation of nerve fibres is, moreover, sometimes propagated 

 to neighboring fibres, so as to excite concurrent movements or sensa- 

 tions ; or it may even extend to neighboring branches, or collateral 

 nerves. Thus, if the sciatic nerve of a frog be detached from the 

 spinal cord, and one of its two chief branches (tibial and peroneal) be 

 cut across, above the point where it enters the muscles, and be stimu- 

 lated by electricity, the muscles supplied by the other branch con- 

 tract, owing to the production of the electrotonic state, not only in 

 those fibres of the trunk of the nerve which belong to the divided 

 branch, but also in those which belong to the undivided branch. 



The phenomena produced by the application of electrical currents 

 to motor nerves, are very complex. The effects are more marked, the 

 further apart, and the more obliquely the poles are brought in contact 

 with the nerve experimented upon. The chief facts hitherto ob- 

 served may be thus summarized. A uniform and constant current 

 produces no muscular contraction ; but when very strong, as already 

 stated, it inhibits the effect of other stimuli. When a constant current 

 is varied in its intensity, then contractions take place at the moment 

 of variation. It is by interrupted currents that contractions are 

 normally produced, and these generally occur both when the current 

 is made, and interrupted, or closed and opened, i. e., as just stated, at 

 the commencement and end of a current, and not during its steady 

 continuance ; but these phenomena are not constant, as we shall im- 

 mediately see. With weak currents, the contraction occurs at the 

 making, closure, or commencement of the current ; with strong cur- 

 rents, at both the making and interruption, i. e., at both the beginning 

 and end of the current. When these interruptions of the current become 

 sufficiently rapid, the muscle is thrown into a state of constant con- 

 traction, or is tetanized. Descending and ascending currents passed 

 along the nerve, are otherwise called centrifugal and centripetal, or 

 direct and inverse currents, because the former pass in the same 

 direction as the volitional stimulus, i. ., from the nervous centre to 

 the muscle, while the latter pursue the opposite course. They each 

 produce peculiar effects in particular circumstances, either according 

 as the current is weak or strong, or according as the limb operated 

 upon, is attached to, or detached from the body, or, lastly, according 

 as the excitability of the nerves and muscles is more or less perfect. 

 The following table shows the results obtained by Hitter and Nobili : 



