

INHERENT CONTRACTILITY OP MUSCULAR FIBRE. 207 



of the other were allowed to remain at rest, it was found after the 

 lapse of two months that the muscles of the exercised limb retained 

 their original size and firmness, and contracted vigorously, whilst those 

 of the other had shrunk to one-half their former size. Though the 

 latter still retained their contractility, there could be no doubt that they 

 would soon lose it, in consequence of the change already far advanced 

 in their physical structure ; .this change not being as rapid in cold-blooded 

 animals, as in Birds and Mammals. 



350. By these and other facts, then, it may be regarded, as com- 

 pletely proved, that the Irritability of Muscles is a vital endowment, 

 belonging to them in virtue of their peculiar structure ; that, so long 

 as this structure is maintained in its normal condition by the nutritive 

 processes, so long is the property capable of being manifested; but 

 that any cause which interferes with the nutrition of a muscle, impairs 

 or destroys its irritability. No cause is so effectual in doing this, as 

 complete disuse; and no means is so sure to produce complete disuse 

 of a muscle, as the division of its nerve, since its being called into 

 exercise in any other way is very improbable ; hence the section of 

 the nerve is almost certain to produce, in time, the loss of the contrac- 

 tility of the muscle. But if a means be devised, by which the muscle 

 may still be * called into action in any other way, as in Dr. Reid's 

 experiment just quoted, its irritability is retained, because its regular 

 nutrition is continued. 



351. We have now to inquire, then, into the circumstances under 

 which this peculiar endowment acts ; or the means by which it may 

 be called into operation, the mode in which the contraction takes place, 

 and the conditions which are necessary for its performance. All Mus- 

 cular Fibre, which has not lost its contractility, may be made to con- 

 tract by a stimulus applied directly to itself; and this stimulus may be 

 of different kinds. The simplest is the contact of a solid substance ; 

 thus we may excite muscular contractions by simply touching the fibre, 

 just as we cause contraction in the tissue of the Dionaea or Sensitive 

 Plant. Most substances of strong chemical action, such as acids and 

 alkalies, will call forth the contractility of muscular fibre, when applied 

 to it ; and the same result is produced by heat, cold, and electricity, 

 the last-named agent being the most powerful of all. The effect of the 

 application of any of these stimuli varies considerably, according to the 

 kind of Muscle on which it is exerted. If we irritate a portion of a 

 muscle composed of striated fibre (any one of the voluntary muscles, for 

 example), the fasciculus of fibres which is touched will immediately 

 contract, and that one only; and the contracted fasciculus will soon 

 relax, without communicating its movement to any other. 



352. If we irritate a portion of non-striated fibre, however, as that 

 of the Alimentary canal, the fasciculus which is stimulated will contract 

 less suddenly, but ultimately to a greater amount ; its relaxation will 

 be less speedy ; and before it takes place, other fasciculi in the neigh- 

 bourhood begin to contract; their contraction propagates itself to 

 others ; and so on. In this manner, successive contractions and relax- 

 ations may be produced through a considerable part of the canal, by a 

 single prick with a scalpel ; a sort of wave of contraction being trans- 



