THE MECHANICAL CHANGES OF MUSCLE 227 



contraction the muscle is, so to speak, after-loaded for the second. 

 The period of contractile stress in a frog's gastrocnemius at the 

 ordinary temperature is only -03 to -04 sec. This sudden jerk 

 is applied through an elastic tissue, the muscle itself, to overcoming 

 the inertia of the weight which has to be raised. If this latter is at 

 all considerable, the moving mechanism is obviously ill-adapted for 

 the purpose. The energy contained in a rifle bullet is very large, but 

 firing a rifle bullet at a door would not be the best way of shutting 

 the door. Even if the door were made of steel the bullet would 

 flatten itself against it and its energy would be transformed for the 



FIG. 62. Contractions of a frog's muscle. Two single twitches are followed 

 by a tetanus, which is almost twice as high as a single contraction. 

 After two more single twitches, the drum was made to rotate more 

 slowly, and single shocks employed, at the same time as the ' after- 

 loading ' was continually increased. It can be seen that the curve 

 obtained in this way is as high as the original tetanus. (V. FREY.) 



most part into a heat, only a small part being utilised in moving the 

 mass of the door. The contractile stress acting through the muscle 

 for a period of -03 sec. is only sufficient to impart a certain velocity 

 to the weight, and therefore to raise it to a certain height. Before 

 the muscle has had time to accomplish its maximum shortening the 

 period of contractile stress has passed away. That this is the case is 

 shown by the fact that if the muscle be after- loaded, so that the lever 

 is raised to the top of the curve of a single twitch, application of a 

 stimulus will make it shorten still more, and by repeated after-loading 

 in this fashion it is possible to make the muscle raise a weight in 

 response to a single stimulus to the same height that it would raise 

 the weight if the stimuli were repeated many times (Fig. 62). In 

 summated contractions the apex of the second contraction occurs 

 rather sooner than would be the case if the second curve had exactly 

 the same course as the first curve. The latent period of the second 

 twitch is also often found to be shorter than that of the first twitch. 

 Both these results might be expected from what we know of the 

 deforming effects of elasticity of the muscle on the graphic record of 

 the mechanical events which occur in the muscle. If summation 

 is to occur at all, the single stimuli must not be applied in too rapid 

 succession. The smallest effective interval depends in any given 

 preparation on the temperature and on the strength of stimulus. It 



