EXPLANATION OF THE PLATES. 



the ridges are marked out into quadrangular spaces, 

 each of which corresponds with a division of the 

 fibrillaa themselves. Now this form of the surface 

 of a striped fibre is especially interesting from the 

 fact of its enabling us to afford a satisfactory ex- 

 planation of the nature of the stria3 themselves. 

 The most recent explanation given of the formation 

 of the striae of the voluntary muscular fibre, and 

 which has been generally adopted, is, that it depends 

 upon the circumstance that the lines on the fibrilke 

 are placed so as exactly to correspond with each 

 other, and that thus a number of smaller lines con- 

 cur to form a larger one, the stria of the entire 

 fibre. Such an exact arrangement of the lines on 

 the fibrillae there is little doubt does really exist, but 

 it is yet insufficient to explain all the characters 

 presented by the muscular stria?. Thus, although 

 the strias are usually strongly marked and broad, 

 yet they have no certain characteristics, either as 

 to position or appearance. In what way then is 

 the muscular stria produced ? A careful examina- 

 tion of a recent muscular fibre with an object glass 

 of the one-eighth of an inch focus will satisfy the 

 observer, that the muscular stria is not a thing of 

 shape and substance itself, but a mere shadow, caused 

 by the ridges into which the surface of the fibre is 

 raised, and which sometimes falls on one side the 

 ridge, sometimes on the other, and frequently in the 

 groove which runs between the ridges, according to 

 the direction of the light, and the focus in which 

 the object is viewed. Of the correctness of this 

 explanation it does not appear to me that there can 

 be a shadow of doubt. 



