THE CHARACTER OF THE CONTRACTION OF MUSCLE 71 



and that of the tuning-fork, it will be found that the muscle does not 

 begin to contract precisely when the shock is passed into it, but a 

 moment thereafter. This period, intervening between the application 

 of the stimulus and the reaction, is designated as the latent period. 

 Hence, a muscle curve really presents three phases, namely a latent 

 period, a period of contraction and a period of relaxation. No visible 

 mechanical energy is liberated during the first, because it is occupied 

 solely by various changes anteceding the actual contraction. 



If the indirect method of stimulation is employed, it may be 

 thought that a large part of the latent period is consumed in the pas- 

 sage of the nerve impulse to the muscle. This contention, however, 

 cannot be considered of much value, because the shifting of the elec- 

 trodes to a place very close to the muscle does not materially shorten 

 this interval, nor does their removal to a more distant point give rise 

 to an appreciable lengthening. It must be evident, therefore, that the 

 conduction of the impulse over the nerve consumes only the briefest 

 possible time and that by far the greatest part of the latent period is 

 consumed in initiating those changes which finally bring the mech- 

 anism of contraction into play. 



As far as the time relationship between these periods is concerned, 

 it should be stated first of all that the duration of a simple contraction 

 of muscle is subject to certain variations which depend upon the char- 

 acter of the muscle tissue and its condition at the time of experimen- 

 tation.^ A perfectly fresh gastrocnemius muscle of the frog completes 

 its contraction in about 0.1 sec, of which 0.01 sec. is taken up by 

 the latent period, 0.04 sec. by the contraction and 0.05 sec. by the 

 relaxation. 



Summation and Fusion of Contractions. — If a second shock is 

 sent into the muscle very shortly after the beginning of its relaxation 

 following the first stimulus, a second contraction will be obtained which 

 is higher than the first. This phenomenon is known as summation of 

 contractions. In quite the same manner, a third contraction may be 

 added to the second and a fourth to the third, and so on, until the 

 relaxations intervening between them become very incomplete and 

 the individual contractions are fused into an incomplete tetanus. If 

 the individual stimuli are now permitted to succeed one another so 

 rapidly that- the relaxations cease to be discernible and the curve as a 

 whole pursues a straight course, the muscle records what is commonly 

 described as a tetanus. 



It should be remembered, however, that the interval between the 

 succeeding shocks cannot be shortened indefinitely, because a point 

 will eventually be reached when the second stimulus loses its effect- 

 tiveness. This fact implies that a certain period must always be 

 allowed to intervene between the different stimulations, otherwise the 

 muscle will be in no condition to receive the succeeding stimulus. In 



^Schultz, Archiv fur Anat. und Physiol., 1897, 22; also see: C. C. Stewart, 

 Am. Jour, of Physiol., iv, 1901, 202. 



