138 MUSCLE. 



and it contains many small granules in its cavity in addition 

 to the nucleus. A transformation of the cell-contents then 

 takes place, the granules gradually disappearing ; the wall of 

 the cell at the same time becoming thicker at the expense of 

 the cavity, so that eventually the latter completely disappears, 

 and the entire secondary cell is converted into a solid cord. 

 The cell-nuclei at first remain whilst this thickening of the 

 cell-wall is going on, and become enclosed by it, rather than 

 pushed into the cavity of the cell. They are at length entirely 

 absorbed. Is, then, the thickening of the wall of the secondary 

 muscle-cell a thickening of the cell-membrane itself, as ap- 

 peared to be the case in cartilage? or is it a secondary deposit 

 upon its inner surface, so that the cell-membrane is chemi- 

 cally and microscopically distinct from the substance, by 

 means of which the secondary cell becomes converted into a 

 solid cord ? The latter is the more usual case in vegetables. 

 The position of the cell-nuclei affords important evidence for 

 the solution of the above question ; for as those bodies, gene- 

 rally at least, lie firmly attached to the inner surface of the 

 cell-membrane, they would be pushed towards the interior by 

 a thickening of the cell-membrane itself, whilst a secondary 

 deposit upon its inner surface, must enclose and fix them 

 there, unless they should become separated altogether from 

 the cell-wall. Now, in muscle, they actually remain lying 

 in the circumference of the fasciculus, as represented by 

 pi. IV, fig. 3, b. This fact, then, renders it probable that the 

 thickening of the wall of the secondary muscle-cells is due only 

 to a secondary deposit. Such a supposition must, however, have 

 been adopted, independent of the argument just raised, since 

 the muscular fasciculi are, as it seems, enclosed by a struc- 

 tureless membrane. The fasciculi have been long described as 

 invested by a sheath, but that investment has been considered 

 to be composed of cellular tissue, and to correspond in the 

 primitive fasciculi to the cellular tissue, by which the larger 

 fasciculi are separated from one another. This membrane 

 seems, however, to have quite a different signification, and 

 to be the cell-membrane of the secondary muscle-cell. It 

 is structureless, very transparent, and appears as a very narrow 

 and sharply-defined border around each primitive fasciculus. I 

 well know how readily such an appearance is produced by a 



