On the structure of cross-striated muscle. 195 



bulated structure of the sarcostyles and especially of the sarcous 

 elements, rather than to their being composed of bundles of fibrils. 

 These appearances will be considered at length when the structure of 

 the wing-muscles is discussed. 



The conclusion, therefore, is that there are reasonable grounds 

 for believing that transversely and longitudinally striated sarcostyles 

 actually pre-exist in muscle. 



Closely interwoven with this point, and at the basis, in fact, of 

 the whole subject is the next question, viz: 



Are the longitudinal lines which are seen in acid- and gold-prepara- 

 tions of muscle actual filaments, or are they the optical expression 

 of planes of separation bekveen the muscle- columns? 



Upon the answer to the question above propounded, the main 

 matter in dispute practically turns. Those who uphold the reticulum- 

 theory naturally answer in one way, those who believe in the sarco- 

 style theory in another. It is, however, a question of fact and not of 

 theory and is capable of receiving a definite solution. 



This solution may be arrived at in three ways, as follows: — 1. By 

 observing the behaviour of the apparent filaments in the longitudinal 

 view of the muscular fibre when the focus is carefully changed. If 

 they are merely filaments and not planes, which are liable to run ob- 

 liquely to the optical axis of the microscope, they may become altered 

 in appearance by raising or depressing the objective, but they will 

 not shift laterally and so pass gradually into others situated in higher 

 or lower horizontal planes: on the contrary, they will be abruptly 

 supplanted by others as the latter come into distinct focus. But they 

 do, in fact, constantly shift gradually with the focus along with the 

 enlargements or dots into which they pass. The dots, since they are 

 admitted by all to be merely the optical sections of the fibres of a 

 transverse network winch has either radial or polyhedral meshes, must 

 of course thus shift: the fine longitudinal lines unquestionably accom- 

 pany them. It is difficult to follow this lateral shifting in the living 

 muscle on account of the extreme indefiniteness of the lines in this 

 condition; but in preparations stained with chloride of gold and in 



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