102 AX AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 jfo second elapsed between the point b, 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 y^-g- 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. 38 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 y^ second, that the rise of the curve occupies 

 about jfo, and the fall about j^-g- second, the total time occupying about -^ 

 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. 39). 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 

 that the muscle protoplasm becomes active before any outward manifestation 

 occurs. That this view is correct has been proved by electrical observations. 



When muscle protoplasm passes from a state of rest to one of action it 

 undergoes an alteration in electrical condition. This change can be detected by 

 the galvanometer (Fig. 62, p. 144) or by the capillary electrometer (Fig. 63, 

 p. 146). Burdon Sanderson s has found that by the aid of the latter instru- 



1 Archiv fur Anatcmiie und Physiologie, 1850, S. 308. 



7 Ibid., 1885, Suppl. Bd., S. 111. 



6 Journal of Physiology, 1898, vol. xxiii. p. 350. 



