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 circum- 

 stances 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 0*1 sec. Since we have no certain means 

 of producing contractions of any given length, except by means of 

 repeated stimuli, it is natural that physiologists 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 con- 

 tinued 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 



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