288 ELECTRO-PHYSIOLOGY CHAP. 



ineffective during the passage of a battery current of medium 

 intensity in the same direction, while it recovers its former 

 efficiency in full as soon as the constant current is broken. 



But it must not be forgotten that even the excised muscle 

 possesses a large capacity of recovery, by which it is enabled to 

 equalise the changes in its substance caused by the excitatory 

 process the more quickly and completely in proportion as the 

 stimulus acts for a shorter time, or the preparation is intrinsically 

 more vigorous. Engelmann's experiments on the ureter show 

 that not only excitability, but conductivity also, appear to be 

 affected after each contraction, i.e. after a relatively short ex- 

 citation, and only recover themselves during the subsequent 

 pause, and that the more quickly in proportion with the original 

 excitability of the preparation. The " refractory " period of 

 contracting cardiac muscle should also be taken into considera- 

 tion. It is, however, clear that if the capacity of recovery in 

 striated muscle is greater, and exceeds that of smooth muscle, in 

 the same proportion as its excitability, diminution of response 

 at the kathode, due to the excitatory process, may have 

 reached considerable proportions after the closure of a strong 

 current, without any necessarily marked after-effect on opening 

 the current, provided that closure lasts for a few seconds 

 only. But it is never possible to exclude a temporary effect upon 

 absolute current-density, of such a kind that with an existing 

 current of a certain magnitude, a superposed positive variation 

 will excite in a lesser degree than before, independent of the 

 fatigue induced by the former. We may therefore assume, that 

 not only the depression of excitability during and after persist- 

 ent polarisation, but also the depression of response at the 

 kathodic fibre-points of a muscle immediately after closure of a 

 strong current, depend essentially upon a local "condition of 

 fatigue," meaning by " fatigue " the total changes in the con- 

 tractile substance of the muscle, produced at the seat of stimula- 

 tion by the excitatory process, which, while they last, prevent, or 

 at least hinder, the rise of a second excitation. We may there- 

 fore conclude as the result of all the preceding data, that the 

 alteration, positive or negative, of excitability at the kathode of a 

 iimxde traversed by current, depend* essential/// upon the state of 

 latent continuous excitation, and its consequent-ex, -ir/i/i'/i /_(//// />//// 

 /lir s/ri'/itfth of the polarising current. 



