222 PHYSIOLOGY 



A simple muscular contraction or twitch, such as that in Fig. 52, 

 produced by a momentary stimulus, consists of three main phases : 



(1) A phase during which no apparent change takes place in the 

 muscle, or at any rate none which gives rise to any movement of the 

 lever. This is called the latent period. 



(2) A phase of shortening, or contraction. 



(3) A phase of relaxation, or return to the original length. 



The small curves seen after the main curve are due to elastic 

 vibrations of the lever, and do not indicate any changes occurring in 

 the muscle itself. From the time-marking below the tracing we see 

 that the latent period occupies about -^ second, the phase of 

 shortening T Q, and the relaxation T fo second. 



Thus a single muscle-twitch is completed in about T V second. 

 It must be remembered, however, that this number is only approxi- 

 mate, and varies with the temperature of the muscle and its condition, 

 being much longer in a fatigued muscle. Moreover, it is almost 

 impossible to avoid some deformation of the curve due to defects of 

 the recording instruments used. Thus the relative period during 

 which no visible mechanical changes are taking place in the muscle 

 must always be shorter than is apparent from a curve obtained by the 

 foregoing method. The elasticity and extensibility of the muscle 

 must prolong the 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 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 instan- 

 taneously. 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 retardation 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 experiments of Sanderson and Burchthe 

 thickening of the muscle at the point stimulated was recorded 

 graphically by photographing the movement on a slit (Fig. 56), 

 behind which was a moving sensitive plate. 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. 57). This figure we can take as the average latent period 

 for the skeletal muscle of the frog at the ordinary temperature of the 

 laboratory (about 16 C.). We shall have occasion later on to consider 

 the changes which occur in the muscle between the application of 



