148 NERVOUS FIBRES. 



aspect, the inner surface of which constantly exhibits cell- 

 nuclei in the very early period of the development of nerve ; 

 but in the somewhat more advanced stage, when the white 

 substance is developed, they are only occasionally found. 2dly. 

 That the white, fat-like substance to which the peculiar appear- 

 ance and distinct outline of the nerves are chiefly referable, is 

 deposited upon the inner surface of this cell-membrane. When 

 this deposit is thick, its double contour (to which the nerve is 

 indebted for its tubular appearance), may be recognised ; this, 

 however, escapes observation when only a thin stratum of 

 white substance is present. Morphologically considered, it 

 therefore corresponds to the peculiar substance of muscle, for 

 that is likewise developed as a secondary deposit upon the mem- 

 brane of the secondary muscle-cell. 3dly. That the rest of the 

 cell-cavity appears to be filled up by a firm substance, namely, 

 the band discovered by Ptemak. There seems to be no struc- 

 ture analogous to this band in perfectly-developed muscles, for 

 there, the secondary deposit, that is, the formation of the pro- 

 per muscular substance, proceeds until the cavity of the 

 secondary cell is completely filled. 



We have thus traced the development of nerve to its per- 

 fect state, without those irregular globules and little cylinders 

 with the dark outlines, (which were mentioned at page 143, 

 as occurring at a middle stage of the development of nerve 

 in addition to the pale fibres and the matured nervous fibres), 

 having proved to be a transitional step in the process. I 

 am inclined to regard them as an artificial product, caused 

 by pressure and the action of water upon the as yet very 

 delicate nerve. If, for example, water penetrate through the 

 cell-membrane by imbibition, the oil-like white substance re- 

 tracts into separate rounded bodies, and the facility with which 

 this takes place is proportionate to its slight degree of con- 

 sistence. This is often seen even in fully-developed nerves ; 

 an entire nerve frequently separates from this cause into sepa- 

 rate globules or little cylinders, which have sharply -defined 

 outlines, so that merely the cell-membrane proceeds uninter- 

 ruptedly, in the form of a pale stripe, from the external wall of 

 one of the dark portions to that of the other. Valentin has 

 given a delineation of such a state of the nervous fibre, (Acta 

 Acad. Leopold. Nat. Curios, vol. xviii, pi. Ill, fig. 7). As the 



