MUSCULAR CONTRACTION. 477 



The experiments of Aeby, which have been repeated and extended by 

 Marey, have shown that when one extremity of a muscle is excited, a con- 

 traction occurs at that point and is propagated along the muscle, in the form 

 of a wave. The estimated rapidity of this wave is 33 to 43 feet (10 to 13 

 metres) per second (Hermann). Applying this principle to the physio- 

 logical action of muscles, Aeby proposed the theory that shortening of the 

 fibres takes place wherever a stimulus is received, and that this is propagated 

 in the form of a wave, which meets in its course another wave starting from 

 a different point of stimulation. Although this view of the physiological 

 action of the muscular fibres is very probable, it can not be assumed that it 

 has been absolutely demonstrated ; but it is certainly more satisfactory and 

 better sustained by experimental facts than any other theory that has yet 

 been advanced. 



Mechanism of Tetanic Muscular Contraction. By a voluntary effort a 

 muscular contraction may be produced, of a certain duration and of a power, 

 within certain limits, proportionate to the amount of force required ; but 

 after a time the muscle becomes fatigued, and it may become exhausted 

 to the extent that it will no longer respond to the normal stimulus. This 

 normal muscular action in obedience to impulses conveyed by motor nerves 

 may be closely imitated by electric stimulation. When a single electric dis- 

 charge is applied to a nerve, there is a single muscular contraction ; but a 

 rapid succession of discharges produces a persistent contraction, which is 

 called tetanic. 



During the passage of a feeble galvanic current through a nerve, there is 

 no contraction in the muscles to which the nerve is attached ; and it is only 

 when the circuit is closed or opened that any action is observed. The inter- 

 rupted galvanic current, the induced current, or a succession of discharges of 

 statical electricity, when they do not follow each other too rapidly, produces 

 a corresponding succession of muscular contractions. As the rapidity of 

 these electric impulses is increased, the individual contractions become less 

 and less distinct, until finally the contraction is persistent. Distinct single 

 contractions occur with ten excitations per second, a partial fusion of the 

 different acts takes place with twenty per second, and a complete fusion, or 

 tetanus, with twenty-seven per second (Marey). When the contraction be- 

 comes continuous, there is an elevation of the line marked on a registering 

 apparatus, showing increased power as the excitations are more and more 

 rapid. This is artificial tetanus ; but it probably is the kind of contrac- 

 tion that occurs in the physiological action of the voluntary muscles. 



It is probable that the normal nervous stimulus in voluntary muscular 

 action is a succession of impulses, which produce a power of muscular con- 

 traction that is proportionate to their rapidity. Vibrations, which are more 

 or less regular, actually occur during the contraction of muscles (Wollaston, 

 Haughton, Helmholtz). Helmholtz, indeed, has recognized a musical note 

 produced by contracting muscles, which exactly corresponds to the number 

 of excitations per second applied to the nerve. This can be heard in the 

 temporal and masseter muscles by filling the ears with wax and causing these 



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