GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 47 



strength, provided these last some little time, but respond to induced currents 

 only when they are very strong. Thus the unstriated muscle which closes the 

 shell of some of the fresh-water mussels, as the Anodonta, gives larger and 

 larger contractions as the duration of the current is increased from one-quarter 

 of a second to three seconds. Much the same is true of the unstriated muscles 

 of the ureters; 1 the battery current must remain closed quite a while for the 

 closing contraction to be called out, the length of time depending upon the 

 strength of the current; and induction shocks have little or no effect unless 

 very strong. Such a comparison makes it evident that the duration of the 

 current is an important element in the influence exerted by electric curreuts 

 on various forms of protoplasm. Unstriated muscles require that the current 

 shall last from one-quarter of a second to three seconds to produce maximum 

 contractions. Striated muscles require that a current shall last 0.001 second 

 (Fick), and even medullated nerves fail to react if the current lasts too short 

 a time. Various forms of irritable tissue can be arranged in series according 

 to their ability to respond to electric currents of short duration, viz. medul- 

 lated nerves, non-medullated nerves, striated muscles, non-striated muscles, 

 and the little-differentiated forms of protoplasm of many of the protozoa. 

 On the other hand these tissues are found to respond in the reverse order to 

 currents which are more prolonged and which change their intensity slowly. 

 It would seem as if the less perfectly differentiated the form of protoplasm, 

 the less its mobility and its susceptibity to passing influences. 



The same form of tissue reacts differently in different animals. For instance, 

 the sluggish striated muscles of the turtle do not respond as well to induced 

 currents as the more rapid striated muscles of the frog. Further, the condition 

 of the tissue at the time is found to have an influence on its irritability and its 

 power to respond to stimuli of short duration. Von Kries reports that nerves, 

 if cooled, react better to slow variations in the intensity of the electric current, 

 and, if warmed, to rapid variations. Under pathological conditions the reac- 

 tion of nerve and muscle to electric currents may become blunted, and, as the 

 tissue degenerates, its power to respond to rapid changes of the electric current 

 is lessened. If a uerve be cut, the part which is separated from the influence 

 of the nerve-cells degenerates. The irritability at first increases and then 

 very rapidly decreases, in from three to four days being wholly lost. As the 

 nerve regenerates, the irritability is recovered very gradually, and the power 

 to respond to the relatively prolonged action of mechanical stimuli is regained 

 sooner than the ability to reply to changes as rapid as those of induced cur- 

 rents. Howell and Huber observed that regenerating nerve-fibres when they 

 have reached the stage resembling embryonic fibres, i. e. are strands of proto- 

 plasm without axis-cylinders, fail to respond to induction currents, though they 

 can be excited by mechanical stimuli. It was found that it is not until the 

 axis-cylinder has grown down into the regenerating fibres that the nerve is 

 capable of responding to induction shocks. 



1 Engclm;mn : Pfluger'a Archiv, L870, Bd. iii. S. 263. 



