70 



A SIMPLE MUSCULAR CONTRACTION. [BOOK i. 



it is found that when a single induction-shock is sent through the 

 nerve the twitch which the muscle gives causes the lever to de- 

 scribe some such curve as that shewn in Fig. 8 ; the lever (after a 

 brief interval immediately succeeding the opening or shutting the 

 key, of which we shall speak presently,) rises at first rapidly but 

 afterwards more slowly, shewing that the muscle is correspondingly 

 shortening, then ceases to rise, shewing that the muscle is ceasing 



FIG. 8. A MUSCLE -CURVE FROM THE GASTROCNEMIUS OF THE FROG. 



This curve, like all succeeding ones, unless otherwise indicated, is to be read 

 from left to right, that is to say, while the lever and tuning-fork were stationary 

 the recording surface was travelling from right to left. 



a indicates the moment at which the induction-shock is sent into the nerve, b the 

 commencement, c the maximum, and d the close of the contraction. 



Below the muscle-curve is the curve drawn by a tuning-fork making 100 double 

 vibrations a second, each complete curve representing therefore one-hundredth of 

 a second. 



to grow shorter, then descends, shewing that the muscle is length- 

 ening again, and finally, sooner or later, reaches and joins the base 

 line, shewing that the muscle after the shortening has regained 

 its previous natural length. Such a curve described by a muscle 

 during a twitch or simple muscular contraction, caused by a single 

 induction-shock or by any other stimulus producing the same effect, 

 is called a curve of a simple muscular contraction or, more shortly, 

 a " muscle-curve." It is obvious that the exact form of the curve 

 described by identical contractions of a muscle will depend on the 

 rapidity with which the recording surface is travelling. Thus if 

 the surface be travelling slowly the up-stroke corresponding to 

 the shortening will be very abrupt and the down-stroke also very 

 steep, as in Fig. 9, which is a curve from a 

 gastrocnemius muscle of a frog, taken with a 

 slowly moving drum, the tuning-fork being 

 the same as that used in Fig. 8 ; indeed with 

 a very slow movement, the two may be hardly 

 separable from each other. On the other 

 hand, if the surface travel very rapidly the 

 curve may be immensely long drawn out, as 

 in Fig. 10, which is a curve from a gastro- 

 cnemius muscle of a frog, taken with a very 

 rapidly moving pendulum myograph, the tuning-fork marking 

 about 500 vibrations a second. On examination, however, it will 



FIG. 9 



