1876.] 



Excitability of Motor Nerves. 



13 



subtraction of the same unit from a weak current. Now wlien the 

 general excitability of the muscle is raised by cutting, the effect is that the 

 muscle is able, both in the case of the ascending make and in that of de- 

 scending break, to afford (as it were) to part with some units of the stimu- 

 lating influence which were previously required to cause adequate stimu- 

 lation. Hence, forasmuch as the sum of such units which it had to spare 

 before cutting was so much less in the case of the make than in that of the 

 break, in the case of the make each unit must have been of a correspond- 

 ingly greater value as a stimulant. Consequently, when both the mini- 

 mals are reduced by cutting, the reduction may take place in a strictly 

 proportional manner ; only, if the proportion has reference to the value 

 of the electrical units as stimulants, it follows, from what has been said, 

 that there will probably be no numerical proportion between the two 

 ratios. 



In favour of this explanation, it is to be remembered that, as already 

 stated, nerve-section produces much more than a proportional effect in 

 the ascending make as compared ^dth the descending break, in respect of 

 increasing the excitability of the muscle to stimuli of short duration. It is 

 as though the comparatively small number of units of electrical intensity 

 by which the minimal make is diminished through nerve-section represents 

 a great actual increase in excitability, when this is estimated by some other 

 method ; or, to turn to the diagram, it seems as though the small distance 

 through which the curve in fig. 2 passes as compared with the curve in 

 fig. 3 really represents an increase of excitability much more important 

 than the curve expresses : it seems as though it is just because the dif- 

 ficulty of ascending (so to speak) increases in so rapid a ratio as its curves 

 approach the zero level, that the steep curve of the descending break 

 terminates at, or below, the point where the much less steep curve of the 

 ascending make begins. This appears to be so, because, on testing the 

 increase of excitability by means of stimuli of short duration, it is found 

 tlfat the relatively low curve in fig. 2 represents what would doubtless be 

 a relatively steep curve, if it were possible to institute the numerical 

 comparisons in the case of stimuli of minimal duration, as it is possible 

 to do so in the case of stimuli of minimal intensity. 



These remarks, however, are only made by way of suggestion ; and I 

 confess that, d priori, I should certainly not have expected so great a dis- 

 proportion to subsist between the curves in figs. 2 and 3. 



§ 4. Sometimes severe section of a tolerably well-curarized muscle 

 will be followed by a development of the breaking contraction treated 

 of in § 2. I attribute this fact to incomplete poisoning of the nerve- 

 elements in the muscle ; for the following experiments prove conclusively 

 that in an uncurarized muscle the development of the breaking contrac- 

 tion after cutting is a purely nervous effect. 



(a) Section of the sciatic nerve just above the knee causes aU the 

 characteristic alterations in the minimal makes and breaks, and this nearly 



