MUSCLES AND PHENOMENA OF CONTRACTION. 129 

 TETANIC CONTRACTIONS. 



But with very rapidly entering stimuli the muscle re- 

 mains in one spasmodic contraction. Such a continued and 

 uninterrupted contraction is spoken of as a tetanus. All 

 the contractions of muscles caused by the will are such 

 tetanic contractions, for the impulses sent along voluntary 

 paths to our muscles are sent at the rate of about ten per 

 second. The fact that a voluntary muscle receiving im- 

 pulses at such a constant rate does not remain in one con- 

 tinuous and constant contraction, is due to the fact that 

 although the rate of the stimuli remains the same, the force 

 of these stimuli may vary, and so the force of the con- 

 traction varies accordingly. 



THE WAVE OF CONTRACTION. 



When a stimulus reaches a muscle the entire muscle 

 does not begin to contract at once, but the contraction pro- 

 ceeds in the form of a wave, beginning at the point where 

 the impulse enters and extending to the opposite ends. 

 Somewhat like a wave in a trough of water caused by the 

 introduction of a pebble, starts at the point where the peb- 

 ble strikes it and proceeds from this point in opposite direc- 

 tion to the ends of the trough. That is to say, the part of 

 the muscle next to the nerve begins to shorten, and then in 

 consecutive order portions further away from it follow. This 

 wave of contraction can be easily noticed on a muscle as a 

 little swelling which runs from the insertion of the nerve to 

 the opposite ends. This rate has been measured, and is 

 in the living muscle (not cut out of the body) about thirty 

 or forty feet per second, while in muscles which have been 

 cut out of the body, on account of their reduced vitality, it 

 is only about fifteen feet per second. In plain muscle tissue 

 the rate of such a wave reaches the low minimum of three 

 to four inches per second. Such a wave of contraction in 

 plain muscular tissue may be easily seen in the creeping 

 peristaltic motion of the intestine. The phenomena here 



described and the times in which they occur apply only to 

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