THE MECHANICAL RESPONSE OF MUSCLE 



207 



the two stimuli is no greater than the effect of a single stimulus. This 

 means that the second stimulus has become ineffective, and this ineffective- 

 ness we must ascribe to the condition set up in the muscle as the result 

 of the first stimulus. For a very short period of time after stimulation a 

 muscle is inexcitable to a second stimulus. The period during which it is 

 inexcitable is known as the refractory period and amounts in skeletal muscle 

 to about -0015 second. The same phenomenon is better marked in certain 

 other excitable tissues, such as the heart muscle, but it seems to be a common 

 property of excitable tissues generally. 



When a loaded muscle is made to record its contractions isotonically we may get 

 summation of effects, though the interval between the stimuli is greater than that 

 which corresponds to the duration of 

 the rise of contractile stress. Thus if 

 the interval is just so long that the 

 second becomes effective just as the 

 contraction due to the first has com- 

 menced to die away, the second con- i 7 



traction seems to start from the point _ 



. , FIG. 67. Muscle curves showing summation of 

 to which the muscle has been raised 



by the first (Fig. 67). By repeating 

 these stimuli in a heavily loaded 

 muscle, the contraction may be made 

 three or four times as extensive as a 

 single twitch. With slow stimuli the 

 summation is however rather mechan- 

 ical than physiological. The period of contractile stress, which lasts only about 

 03 second, is so short that it has no time to raise the weight to the maximum height 

 before it has passed away. 'This 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 the 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 if excited by a 

 series of stimuli. This 

 mechanical factor in sum- 

 mation is shown in Fig. 

 68. It will be noted 

 however that the tetanus 

 is not a steady one and 

 is probably due to stimuli 

 FIG. 68. Contractions of a frog's muscle. Two single twitches repeated at intervals of 

 are followed by a tetanus, which is almost twice as high as a a bout * of a second. 



stimuli, r and r'. the points at which the 

 stimuli were sent into the nerve. From the 

 first stimulus alone the curve abc would be 

 obtained. From r' the curve def is obtained. 

 These two curves are summaled to form the 

 curve aghik when both stimuli are sent in at 

 the interval r r'. 



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 were increased to 50 or 



the rate of 



If 



stimulation 



increased. It can be seen that the curve obtained in this way 

 is as high as the original tetanus. (V. FREY.) 



100 per second, a tetanus 

 . e i& ^w, .,. v ,. A - Jai . y would be produced and 



the curve would be prob- 

 ably twice as high as that represented in the figure. We thus see that for the over- 

 coming of a resistance a single twitch is not economical. It is doubtful whether any 

 contractions of muscles which occur in the body are other than tetani of varying 

 duration. 





TEMPERATURE. Speaking generally, the effect of warming a muscle 



