THE PHYSIOLOGY OF MUSCLE 



485 



another abscissa line 5 cm. below the other. When the drum has rotated 

 at same speed as before to the place of the first curve shift the rocker, 

 thus sending a galvanic make-shock through the muscle. Compare the 

 two curves. The former curve, made with the exceedingly sudden induc- 

 tion break shock will be found to be more acute than that made by the 

 continuous galvanic current. 



This briefly sustained contraction of the cross-striated muscle (it is 

 longer in smooth muscle) when stimulated with the galvanic shock is 

 doubtless due to a condition of brief electrotonus, which has no existence 

 when the muscle is stimulated with the almost instantaneous induced 

 shock. The reason that the myogram is broader on top (indicating a 

 sustained contraction) is that the stimulus in case of galvanism lasts 

 longer than it does in case of electricity produced by induction. To 

 stimulate a cross-striated muscle requires a duration of a galvanic current 

 of at least 0.001 second, while to have maximal contraction of a smooth 

 muscle the electricity must be applied from 0.25 to 5 seconds. 



FIG. 270 



Threshold Stimulus, Galvanic 



Threshold Stimulus, Induced 



These curves show in general the relations of the degree of contraction of cross-striated 

 (frog's gastrocnemius) muscle to various intensities of stimulus, beginning at the threshold and 

 ending at the maximum. The left-hand set of curves was made with galvanic, the right-hand 

 set with induced, electricity. To be read from left to right. Constant load of 10 gms. 

 Intervals between contractions about thirty seconds. Reduced. 



Expt. 41. The Threshold and the Maximum Stimulation. (Apparatus : 

 Myograph, rheocord, inductorium.) (A) Galvanism. Place the block of 

 the rheocord so that the make of the current just does not produce con- 

 traction. By gradually moving the slider, increase the current with the 

 key closed until the make of the current just barely causes contraction. 

 Make a record (straight vertical line) of the contraction on the drum 

 while stationary; move the block a few centimeters so as to increase the 

 stimulus, turn drum 0.5 centimeter by hand and make another record 

 and so on. A series of contractions up to the maximum of the cell is 

 thus produced. 



(B) Induction. Repeat the experiment, using the induction-coil 

 instead of the rheocord, cutting out (with short-circuiting key on the 

 secondary coil) all the make -.shocks. Compare this series with that 

 made with make galvanic shocks. Observe the threshold; the increase 

 in contraction; and that a place is soon reached above which no strength 

 of current would increase the contraction, but rather injure the muscle. 



