SECTION IX 

 VOLUNTARY CONTRACTION 



THE whole of our analysis of the processes accompanying the contraction 

 of a skeletal muscle has so far had reference merely to the contractions 

 evoked by artificial stimuli, mainly electric. These contractions have 

 either been the simple twitch, with a duration of about one-tenth of a second, 

 evoked by a momentary stimulus, or the tetanus, a continued contraction 

 composed of a number of single twitches, summated and fused together. 

 Under normal circumstances the contraction of skeletal muscles is brought 

 about either reflexly, or in response to some stimulus descending from the 

 cerebral cortex, the so-called ' voluntary contraction.' These contractions 

 may have a duration of almost any extent. The quickest contractions 

 carried out by man have a duration of about 0-1 sec. Considerable effort 

 and training are required to reduce a muscular movement to this degree, 

 and nearly all contractions, even the rapid ones, last considerably over 

 O-l sec. Since we have no certain means of producing contractions of any 

 given length, except by means of repeated stimuli, it is natural that physiolo- 

 gists have regarded voluntary contractions as similar to the artificial tetanus, 

 and as, like this, composed of fused single contractions, and have endeavoured 

 to determine the number of contractions per second, i.e. the natural rhythm 

 of the tetanus. If, however, every muscular contraction in the body is to 

 be regarded as of the nature of a tetanus, effected by rapidly repeating 

 stimuli sent down the motor nerve from the central nervous system, we 

 must assume a similar discontinuity for the process underlying the normal 

 tone of muscles, and for the continued contraction of unstriated muscles, 

 e.g. of the arteries. Is this discontinuity of muscles really essential for the 

 production of a prolonged contraction ? So far as our present knowledge 

 of the intimate nature of muscular contraction goes, it would seem quite 

 possible that the continuous state of contraction is dependent on a continuous 

 evolution of energy in the muscle. We have seen reason to regard the 

 chemical processes in a contracting muscle as presenting two phases, namely, 

 (1) the production of a substance which increases the osmotic pressure 

 within the sarcous elements, or raises the surface tension of the ultimate 

 contractile elements of the muscle, thus causing a shortening and thickening 

 of those elements ; and (2) the further change of this substance into one 

 which can escape by diffusion, or into a substance with a low surface tension, 

 so that now the muscle relaxes and can be stretched by any extending 



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