THE MUSCLES. 



547 



necessary to use very rapid breaking of the current, some instrument 

 must be employed for that purpose. The first instrument used in 

 making myograms was that of Helmholtz. 



Simple Contraction. If .a single induction shock be applied to 

 a muscle there will result a simple muscular contraction; that is, the 

 muscle will respond by a quick contraction, with return to its former 

 relaxed condition. This contraction, when graphically shown, is 

 termed a simple muscle-curve: 



MUSCLE-CURVE, OR MYOGRAM. If the muscle-curve of a single 

 stimulus be analyzed, it will be seen to be composed of various ele- 



Fig. 191. A Muscle-curve Obtained by Means of the Pendulum 

 Myograph. ( FOSTER. ) 



To be read from left to right. 



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

 6, The commencement; c, the maximum; and d, the close of the contraction. 

 The two smaller curves succeeding the larger one are due to oscillations of the 

 lever. 



Below the muscle- curve is the curve drawn by a tuning-fork making 180 

 double vibrations a second, each complete curve therefore representing Yiso * a 

 second. It will be observed that the plate of the myograph was traveling more 

 rapidly toward the close than at the beginning of the contraction, as shown by 

 the greater length of the vibration-curves. 



mts, as follows : (1) period of latent stimulation, (2) period of con- 

 tion } and (3) period of relaxation. 



Latent Period. The significance of this term is that the muscle 

 >erimented with does not respond at the precise moment when the 

 timulus is applied to it. The response comes later about 1 / 100 of 

 second. During the latent period there is no apparent change 

 mrring within the muscle. The latent period may be modified by 

 ised stimulus and heat, when it becomes shortened ; fatigue and 

 Id lengthen the time. The latent period of unstriped muscle may 

 be as long as one or two seconds. 



Contraction Period. The muscle-curve comprises two periods: 

 it of the ascent and that of the descent of the muscle. The ascent 



