514 MUSCLE-NERVE PHYSIOLOGY 



is called a myogram. The character of the myogram, and therefore the facts 

 revealed by it, are dependent on whether or not the record is made on a rapidly 

 moving recording surface. If the myogram is made on a recording surface 

 that is standing still, then it shows merely the extent of shortening of the 

 muscle. The amount of shortening for a given muscle will depend on a series 

 of conditions, such as nutrition, load, temperature, etc., all of which will 

 be discussed presently. 



When the record is made on a rapidly moving drum or on the pendulum 

 myograph, it is revealed that the simple contraction occupies a definite per- 

 iod of time with well-marked periods or phases. Although the stimulus may 

 be practically instantaneous, the contraction lasts a considerable fraction of a 

 second, in the frog's gastrocnemius about o. i of a second. 



FIG. 322 Record of a Simple Contraction of the Gastrocnemius of the Frog. Time 

 in o.oi second. Si, Moment of stimulation. Record taken on a rapid drum that was 

 provided with an automatic key. 



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

 by the vertical line St, there is an interval before contraction commences. 

 This interval, termed the latent period, when measured by the number of vi- 

 brations of the tuning-fork directly beneath, is found to be about o.oi of a 

 second. The latent period is longer in some muscles than in others and 

 differs also according to the condition of the muscle and the kind of stimulus 

 employed. During the latent period there is no apparent change in the 

 muscle. The second part of the record shows the contraction phase proper. 

 The lever is raised by the sudden shortening of the muscle. The contrac- 

 tion is at first very rapid, but then progresses more slowly to its maximum. 

 It occupies on an average o . 04 of a second in the frog's gastrocnemius. The 

 third stage is the relaxation phase. After reaching its highest point, the lever 

 begins to descend, in consequence of the elongation of the muscle. At- first 

 the fall is rapid, but it then becomes more gradual until the lever reaches the 

 abscissa or base line, when the muscle has attained its precontraction length. 

 The stage occupies o . 05 of a second. Usually after the contraction proper 

 is over the lever oscillates below and above the base line in a series of dimin- 

 ishing waves, the elastic rebound following movement of the simple contrac- 



