THE MECHANICAL CHANGES OF MUSCLE 



199 



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

 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 



Fia. 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'. 



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

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

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

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

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



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. In the experiments 

 of Sanderson and Burch 

 the thickening of the muscle 

 at the point stimulated was 

 recorded graphically by photo- 

 graphing the movement on a FIG. 58. Photographic record of muscle-twitch . 



slit (Fio- 57^ behind which (B ' SANDERS N.) The upper curve is the move- 

 rig. 01), I wnicn ment of the muscle> the middle curve the signal 



Was a moving sensitive plate. showing the moment of excitation, and the lower 



_ . i 1 _ t f t l 1 '1 _ J __ *>/\f\ A 



Thus avoiding all instrumental 

 inertia, and diminishing the 

 inertia of the muscle to a minimum, the mechanical latent period was 

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

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

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



* that of a t in g- fork vibrating 500 times 





