MUSCLE 575 



stance 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. 630) 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 contrac- 

 tion springs up first of all about the middle of each mus- 

 cular 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 in- 

 volved in some phase or other of the contraction. And this 

 is the case even when the end of ^i 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 of the kind 

 called 'tonic.' An artificial stimulus, mechanical or electrical, 

 causes, after a long latent period, a very definitely-localized contrac- 

 tion 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 seem to 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 tonic contraction of smooth 

 muscle. The contraction both of unstriped and of cardiac muscle 



