CHAP. VII.] MUSCULAR ACTION. 17'J 



Of the Differences between the minute Movements of Muscle in Passive 

 and Active Contraction. 



In Passive Contraction. It is perhaps impossible, in the higher 

 animals, to observe the nature of the microscopic movements occur- 

 ring in muscle in its ordinary state or during its passive shortening ; 

 but, in the lower and smaller forms of life, this may sometimes be ac- 

 complished. It may always appear doubtful, however, whether any 

 contraction that may be here witnessed be entirely of the passive 

 kind, and consequently the movements here noticed are not worthy 

 of implicit reliance. But it is more easy, and quite as satisfactory, 

 to bring a muscle under inspection, which is still in situ and in 

 equilibrium with its antagonists : in such, contractile force is being 

 still exerted, though its full effects are prevented from taking place. 

 This may be done in various small animals : perhaps the tail of 

 small fish, or of the tadpole of the common frog, is the best adapted 

 for the purpose. In the latter, deprived of its integument, we have 

 obtained such a view, and have found the contraction to be quite 

 uniform throughout, the transverse stripes being stationary and 

 equidistant. This is nothing more than might have been expected 

 on a priori grounds. The contraction, being the effect of the pas- 

 sive exercise of the property shared equally by all parts of the 

 tissue, would be equal in equal masses; and, as the elementary 

 fibres are of precisely equal width and substance from end to end, 

 no part of them could predominate in action as long as only the 

 equable stimulus coincident with their natural state of tension were 

 applied. It may be concluded, therefore, that passive contraction 

 is attended by a movement absolutely uniform throughout the 

 whole mass of an elementary fibre, or of a muscle. 



In Active Contraction, the case is far otherwise, as may now be 

 considered proved by a considerable body of evidence. 



It might be argued, prior to direct proof, that active contraction 

 must 1)0 partial, at least at its commencement ; because the stimuli 

 which occasion it cannot, in their very nature, be applied to every 

 particle of the fibre at one and the same instant of time. 



Certain features of the phenomena witnessed under the micro- 

 scope in fragments removed from the body, and contracting in 

 water, have a close bearing on the present question. It has been 

 already said, that such contractions are uniformly partial : but they 

 present two further varieties, either remaining in the part where 

 they first occur, or leaving it as they advance to others in the 



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