CONTRACTION OF MUSCLE. 449 



their natural sequence, what is the actual physical cause of 



the shortening of the muscular fibril? All that we have extensive mus- 



thus far said can Ibe received at the best as only a statement cuiar contrac- 



f tion by slight 



of a succession or order of facts. To say that that shorten- muscular 

 ing is the direct consequence of loss of material involves us waste * 

 at once in the inquiry whether it be possible, through the destruction of 

 so small an amount of material as we know to occur, that any thing like 

 the required extent of motion could be produced. Could a muscle be 

 made to shorten several inches, and, upon these principles, lose only an 

 insignificant amount of weight, the shortening being nevertheless the 

 consequence of that loss of weight or destruction of substance ? To an- 

 swer this inquiry, we have, in the first place, to recall the fact that a 

 whole muscle is never in contraction at once, but only an insignificant 

 portion thereof, one bundle of fibrils after another taking up the action in 

 succession, and each particular fibril undergoing change, not throughout 

 its whole length, but only in isolated portions here and there. We have, 

 moreover, to recall the insignificant weight of these fibrils, for a simple 

 computation will show that thirty thousand of them a foot long weigh 

 only a single grain. To these recollections we should add the intense 

 energy of the molecular force of attraction, as displayed at such distances 

 as those which we have here under consideration distances which we 

 may regard as being virtually inappreciable, and these recollections place 

 the problem in its true light, and set it in its proper attitude before us. 



For it is capable of demonstration that muscular contraction ensues as 

 the direct consequence of destruction of muscular substance, and that a 

 great linear extent of movement may be accomplished by the removal of 

 an insignificant amount of substance. If 100,000 fibrils lost one third 

 of their entire substance -a thing which, of course, could scarcely take 

 place the diminution of weight would only amount to a single grain. 

 Our conception of this action may perhaps be rendered clearer by an il- 

 lustration. If we had an iron thread of excessive tenuity, illustration of 

 composed, for instance, of a single row of iron atoms set end ofa^usTte^ 

 to end, and could, by suitable processes, effect the removal, fibre. 

 here and there, of atoms in the line, an instantaneous contraction would 

 be the result, the thread shortening in proportion to the number of atoms 

 removed, but shortening with an energy commensurate with the cohesive 

 force of the iron itself, and yet ready to return to its original length the 

 moment that fresh iron atoms present themselves to be introduced in the 

 place of the abstracted ones ; and so with muscular fibre, the molecular 

 force of cohesion developed here and there by the removal of tissue is to 

 be measured only by the cohesion of the fibre, though the loss of mate- 

 rial which may have been the cause of that force coming into play may 

 be very small indeed; nor does the quickness of relaxation present any 



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