178 LOCOMOTION. [CHAP. vn. 



aggregations of particles bearing those particular forms, in pro- 

 ducing the phenomena of contraction. A contraction is never 

 limited to a particular number of discs or fibrillse, and is never 

 accurately bounded by the interval between two discs. It constantly 

 happens, that, at the edge of the contracted part, several discs are 

 only partially engaged in it. A contraction, generally, when com- 

 mencing at the broken end of a fibre, occupies its whole width 

 there ; but, when it commences at the border of the fibre, it may be 

 confined to a portion of many discs: and, further, the contractile 

 force is never exerted along the whole length of a fibre or fibrilla at 

 once. A contraction excited in an elementary fibre by the contact 

 of a hair extends into the mass equally in all directions, as we 

 might suppose it would do, if the mass were homogeneous. 



An attentive study of these interesting phenomena will lead to 

 the conviction, that, in the bare fact of contraction, the build of the 

 fibre is an item of no importance whatever : the exquisite symmetry 

 displayed in the apposition of its component particles is, as it were, 

 disregarded and overlooked; while the whole process is to be referred 

 to the material itself, the ultimate tissue, whose property is con- 

 tractility. This property appears to reside both in the particles and 

 the substance connecting them. 



The ultimate movements, therefore, on which contraction de- 

 pends, whatever they may consist in, are molecula, and far beyond 

 the reach of sense. 



It will be perceived, that this view of the subject is the only one 

 which can harmonize the fact of contraction in voluntary muscle 

 with the same phenomenon in structures which have no complicated 

 internal arrangement of particles, as, for example, in the unstriped 

 fibre; and the contractility manifested by fibrine, immediately 

 after coagulation, is a property too nearly allied to the con- 

 tractility of muscle (a form of fibrine) not to give it additional 

 credibility. 



In regarding contractility, therefore, as a property of the living 

 muscular fibre in general, it is meant that it resides in it as a pro- 

 perty without which it would not be muscle ; and in such a manner, 

 that no particle, however microscopic, can be detached from muscle 

 which does not of itself, and independently of the rest, possess this 

 property as long as it possesses vitality. 



