190 



THE MECHANICAL CHANGES OF MUSCLE 



by the foregoing method. The elasticity and extensibility of the muscle , 

 prolong the apparent latent period, since the first effect of contraction of any 

 part of the muscle will be to stretch the adjacent part, and only later to 



FIG. 57. Burdon Sanderson's method for photographic record of muscle-twitch. 

 The exciting shock is sent into the muscle by the wires d and d'. 



move the tendon to which the lever is attached. Thus if we have a weight 

 supported by a rigid wire, and suddenly pull the upper end of the wire so as to 

 raise the weight, the latter will rise instantaneously. If, however, the 

 weight be suspended by a piece of elastic, it will not follow the pull exactly, 

 but will lag behind, the first part of the pull being occupied with stretching 

 the india-rubber, and only 

 when this is stretched to a 

 certain degree will the weight 

 begin to rise. The same re- 

 tardation of the pull would be 

 observed if, instead of india- 

 rubber, we used a piece of 

 living muscle. 



It is possible to obviate 

 this instrumental inertia by 

 employing solely photographic 

 methods for the record and 

 magnification of the muscle- 

 twitch. Thus in the experi- 

 ments of Sanderson and Burch 

 the thickening of the muscle 

 at the point stimulated was 

 recorded graphically by photo- 

 graphing the movement on a Pro. 58. Photographic record of muscle-twit. h. 

 /TO _ , , . . (B. SANDERSON.) The upper curve is the move- 



Silt (Fig. 57), behind Which menfc O f t h e muscle, the middle curve the signal 

 was a moving Sensitive plate. showing the moment of exc itatkm and 

 _.. curve is that of a tuning-fork vibrating 500 



Ihus avoiding all instrumental a sec ond. 



inertia, and diminishing the 



inertia of the muscle to a minimum, the mechanical latent pen 



found to be only 0-0025 second (Fig. 58). This figure we can take as i 



average latent period for the skeletal muscle of the frog at the ordinary t 



perature of the laboratory (about 16 C.). We shall have occasion later on 



