150 NERVOUS FIBRES. 



a simple cell, and since the simple cell-membrane continues to 

 exist distinct from its secondary deposits, and from the cell- 

 contents, it is quite conceivable that fibres may be generated 

 in the secondary deposits or in the cell- contents, as they are in 

 muscle, although we have as yet no evidence of the fact ; but 

 these fibres could no more issue out free from the white 

 nervous fibre, than the primitive fibres of muscle could from 

 secondary muscle-cell, because, in order to do so, they must 

 necessarily rupture the cell-membrane of the secondary cell. 

 These subdivisions, therefore, so far as the structure from 

 whence they issue corresponds to an ordinary nervous fibre, 

 and is not merely a fasciculus of very minute secondary nerve- 

 cells, cannot be a mere appearance, nor anything but actual 

 divisions, a simple secondary nerve-cell becoming elongated into 

 several minute fibres, in a manner analogous to that which 

 we have witnessed in the fibre-cells, (see page 115.) The 

 nerves in the tail of the tadpole may therefore be described 

 as terminating by the nervous fibres, that is, the secondary 

 cells becoming split in different directions after the manner 

 of fibre-cells or stellate cells. In the memoir before alluded 

 to, I have noticed some swellings upon the pale nervous fibres 

 in the tail of the tadpole. They have a double signification ; 

 some which are marked off from the rest of the fibre by a 

 sharply- defined outline are the nuclei of the cells, from which 

 the fibres have been generated ; the majority, however, which 

 pass into the fibre without a well-defined contour, as gene- 

 rally occurs at situations where the fibres divide and diverge 

 towards different sides, are the bodies of the original cells, 

 which (especially when they become elongated at different 

 parts into fibres) remain somewhat thicker than the prolonga- 

 tions themselves; the pigment -cells, pi. II, fig. 9 a, exhibit 

 this appearance. 



b. Gray or organic nervous fibres. The gray cords, which, 

 according to the researches of Retzius and J. Miiller, are derived 

 from the sympathetic nervous system, and mingled with the 

 cerebrospinal nerves in which they sometimes pursue a long 

 isolated course, owe their gray appearance, according to the in- 

 vestigations of Remak, " to the peculiar structure of the primi- 

 tive fibres, which arise in the ganglia. They are not tubular, 



