MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 733 



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 suffi- 

 cient. 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 gastrocnemius, particularly 

 in spring after the winter fast. It is not a tetanus and is not propa- 

 gated 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 contrac- 

 tion. 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 measure- 

 ment of the rate at which the negative variation (p. 797) 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 temperature 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. 



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 discontinuous that is, 

 composed of summated contractions like a tetanus; it appears to be a 

 greatly-prolonged simple contraction. An artificial stimulus, mechani- 

 cal 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 rhythmical contraction of 

 the heart is not a tetanus has already been seen. It is a simple con- 

 traction, intermediate in its duration and other characters between the 

 twitch of voluntary muscle and the contraction of smooth muscle. The 

 contraction both of unstriped and of cardiac muscle is lengthened and 

 made stronger by distension of the viscera in whose walls they occur, 

 just as a skeletal muscle contracts more powerfully against resistance. 



