MUSCLE-NERVE PHYSIOLOGY. 513 



dulum carries a smoked glass plate upon which the writing lever of a myo- 

 graph is made to mark. The opening or breaking shock is sent into the 

 nerve-muscle preparation by the pendulum in its swing opening a key (fig. 

 327, C.) in the primary circuit. 



Single Muscle Contractions. The tracing (muscle curve) ob- 

 tained of a single muscle contraction or twitch is seen in fig. 328, and 

 may be thus explained. 



The upper line (m) represents the curve traced by the end of the lever 

 in connection with a muscle after stimulation of the muscle by a single 



Fig. 828. Muscle-curve obtained by means of the pendulum myograph. s, indicate* the 

 exact instant of the induction shock ; c, commencement ; and m #, the maximum elevation of 

 lever; t, the line of a vibrating tuning-fork. (M. Foster.) 



induction-shock: the middle-line (?) is that described by the marking- 

 lever, and indicates by a sudden drop the exact instant at which the 

 induction-shock was given. The lower wavy line (t) is traced by a 

 vibrating tuning-fork, and serves to measure precisely the time occupied 

 in each part of the contraction. 



It will be observed that after the stimulus has been applied, as indi- 

 cated by the vertical line s, there is an interval before the contraction 

 commences, as indicated by the line c. This interval, termed (a) the 

 latent period, when measured by the number of vibrations of the tun- 

 ing-fork between the lines s and c, is found to be about y^- sec. The 

 latent period is longer in some muscles than in others, and differs also 

 according to the condition of the muscle, being longer in fatigued mus- 

 cles, and the kind of stimulus employed. During the latent period there 

 is no apparent change in the muscle. 



The second part is the (b) stage of contraction proper. The lever 

 is raised by the sudden contraction of the muscle. The contraction is at 

 first very rapid, but then progresses more slowly to its maximum, indi- 

 cated by the line m x, drawn through its highest point. It occupies in 

 the figure yfo sec. (c) The next stage, stage of elongation. After 

 33 



