MUSCLE 659 



power of most obstinate contraction. Richet tetanized one for 

 over seventy minutes, and another for an hour and a half, before 

 exhaustion came on, while a tetanus of a single minute exhausted 

 the muscles of the crayfish's tail. The gastrocnemius of a summer 

 frog kept up for twelve minutes, and a tortoise muscle for forty 

 minutes. 



Continuous stimulation is not always necessary for the production 

 of continuous contraction ; in some conditions a single stimulus is 

 sufficient. A blow with a hard instrument may cause a dying or 

 exhausted, and in thin persons even a fairly normal, muscle to pass 

 into long-continued contraction. This so-called ' idio-muscular ' 

 contraction seems to depend, in part at least, on the great intensity 

 of the stimulus. It can sometimes be obtained in the frog's gastroc- 

 nemius, particularly in spring after the winter fast. It is not a 

 tetanus and is not propagated along the muscular fibres, as an 

 electrical tetanus is, but remains localized at the spot where it arises. 

 Similar non - tetanic contractions have already been mentioned, 

 such as the tonic contraction during the passage of a strong voltaic 

 current and the sustained veratrine contraction. Ammonia causes 

 also a long but non -tetanic contraction, and this, too, does not spread 

 when the substance has acted only on a portion of the muscle. The 

 contraction force of all these tonic contractions, as measured by the 

 resistance necessary to overcome or prevent them, is less than the 

 contraction force in electrical tetanus (Schenck) . 



The rate at which the wave of muscular contraction travels may 

 be measured by stimulating the muscle at one end, and recording, 

 by means of levers, the movements of two points of its surface as 

 far apart from each other as possible. Time is marked on the 

 tracing by means of a tuning-fork, and the distance between the 

 points at which the two curves begin to rise from the base-line 

 divided by the time gives the velocity of the wave. Another 

 method is founded upon the measurement of the rate at which the 

 negative variation (p. 719) passes over the muscle, this being the 

 same as the velocity of the contraction-wave. In frog's muscle it 

 is about three metres a second, or six miles an hour. Rise of tem- 

 perature increases, fall of temperature lessens it. 



When a muscle is excited through its nerve, the contraction 

 springs up first of all about the middle of each muscular fibre where 

 the nerve-fibre enters it, and then sweeps out in both directions 

 towards the ends. But so long is the wave, that all parts of the 

 fibre are at the same time involved in some phase or other of the 

 contraction. And this is the case even when the end of a long 

 muscle like the sartorius is artificially stimulated. 



The wave of contraction in unstriped muscle lasts a relatively 

 long time at any given point, and in tubes like the intestines and 

 ureters, the walls of which are largely composed of smooth muscle 

 arranged in rings, the wave shows itself as a gradually-advancing 

 constriction travelling from end to end of the organ. There is no 

 evidence that the contraction of smooth .muscular fibres is discon- 

 tinuous that is, composed of summated contractions like a tetanus ; 

 it appears ^to be a-greatly-prolonged simple contraction. An artificial 

 stimulus, mechanical or electrical, causes, after a long latent period, 

 a very definitely-localized contraction in a rabbit's ureter, which 

 slowly spreads in a peristaltic wave in one or both directions along 

 the muscular tube. Here, as in the cardiac muscle, the excitation 

 passes from fibre to fibre, while in striped skeletal muscle only the 

 fibres excited directly or through their nerves contract. That the 



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