in ELECTRICAL EXCITATION OF MUSCLE 183 



a still shorter duration of closure being sufficient to excite the 

 latter. 



Summing up the results of the preceding observations, we 

 may say that under all conditions a current of given strength 

 must traverse the muscle for a perceptible time, in order to 

 bring it from a state of rest into that of maximal excitation, 

 corresponding with the intensity of the same current. If the 

 cause of excitation, i.e. the current, acts for too short a time, a 

 weak contraction only will ensue, because the new state cannot 

 develop itself fully ; with still shorter duration of current, the 

 effect is altogether wanting, because the stimulus does not act 

 long enough to induce in any perceptible degree those changes 

 in the muscular substance which are the fundamental cause of 

 contraction. The time required varies within a very wide range 

 in different muscles with quick reaction, but is, generally speaking, 

 greater, as the period of contraction is more sluggish. 



If we may conclude from the above that the excitatory 

 process is caused by the current, not merely at the moment of 

 its commencement, but also during its pasmyc, this is still more 

 certain from a closer investigation of the changes of form in a 

 muscle durini/ jti'i-xixfait closure of current. We have already 

 pointed out in smooth molluscan muscle that it may, under these 

 conditions, remain as long as a minute in unbroken, persistent 

 contraction. The magnitude of this " persistent closure con- 

 traction " increases up to a certain limit with the strength of the 

 exciting current, but the effect per se is quite evident at all 

 working grades of intensity ; indeed, it may be said that the 

 persistent closure contraction is, generally speaking, the single 

 form of contraction in smooth molluscan muscle that corresponds 

 with persistent closure. If the reaction of striated muscle is 

 compared under the same conditions, there are noticeable differ- 

 ences. We have already found that below a certain limit of 

 current intensity a single " twitch " is alone provoked at closure, 

 the muscle shortening rapidly, and elongating again almost as 

 quickly, even when the circuit remains closed. When in any 

 given case the closure twitch has reached its maximum, a further 

 increase of current intensify produces no increment in height of 

 contraction, but there are certain changes in the form of the 

 contraction curve, which express the persistent shrinking of the 

 muscle during the entire passage of the current. 



