GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 101 



Myogram of Simple Muscle-contraction.- -The rate of the movement of the 

 muscle during every part of its contraction can be readily determined by com- 

 paring the record it has drawn with that of the chronograph. 



Figure 35 is the reproduction of a single contraction of a gastrocnemius 

 muscle of a frog. The rise of the curve shows that the contraction began 

 comparatively slowly, soon became very rapid, but toward its close was again 

 gradual; the relaxation began almost immediately, and took a similar course, 

 though occupying a somewhat longer time. The electric current which 

 actuated the chronograph was made and broken by a tuning-fork which 

 made 200 complete vibrations per second, therefore the spaces between the 

 succeeding peaks of the chronograph curve each represents 0.005 second. A 

 comparison of the movements of the muscle with the tuning-fork curve 

 reveals that about -^ second elapsed between the point 6, at which the muscle 

 curve began to rise, and c, the point at which the full height of the contraction 

 was reached, and that about ^ second was occupied by the return of the 

 muscle curve from c to point d, at the level from which it started. The muscle 

 employed in this experiment was slightly fatigued, and the movements were 

 in consequence a little slower than normal. 



Latent Period. The time that elapses between the moment that a stim- 

 ulus reaches a muscle and the instant the muscle begins to change its form is 

 called the latent period. In the experiment recorded in Fig. 35 the muscle 

 received the shock at the point a on the curve, but the lever did not begin to 

 rise until the point b was reached. The latent period as recorded in this ex- 

 periment was about 0.006 second. The latent period and the time relations of the 

 muscle-curve were first measured by Helmholtz, who introduced the use of the 

 myograph. 1 Helmholtz concluded from his experiments that the latent period 

 for a frog's muscle is about -^-^ second, that the rise of the curve occupies 

 about Y^-, and the fall about j^- second, the total time occupying about -fa 

 second. These rates can be considered approximately correct, excepting for 

 the latent period, which has been found by more accurate methods to be con- 

 siderably shorter. Tigerstedt connected a curarized frog's muscle with a myo- 

 graph lever, which was so arranged as to break an electric contact at the 

 instant that the muscle made the slightest movement ; the break in the electric 

 circuit was recorded on a rapidly revolving drum, by an electro- magnet similar 

 to the chronograph. By this means he found the latent period of a frog's 

 muscle may be as short as 0.004 second. Tigerstedt 2 did not regard this as 

 the true latent period, however ; he expressed the belief that the muscle proto- 

 plasm must have begun to respond to the excitation much sooner than this. 

 The contraction of the whole muscle is the result of a shortening of each of the 

 myriad of light and dark disks of which each of the muscle-fibres is composed 

 (see Fig. 36). The distance to be traversed by the finest particles of muscle- 

 substance is microscopic, hence the rapidity of the change of form of the whole 

 muscle. Even such a change would require time, however, and it is probable 



1 Archiv fur Anatomic und Physiologic, 1850, p. 308. 



2 Ibid., 1885, Suppl. Bd., p. 111. 



