NERVOUS FIBRES. 1 15 



in their independent state, is very difficult, from the circum- 

 stance of our being unable at that period to distinguish between 

 them and the surrounding tissues ; for a whole organ is then 

 composed entirely of independent cells, which have not as yel 

 undergone any transformation. It is true I saw an independent 

 cell, furnished with a nucleus, which seemed to have separated 

 from the nervous cord, in one of the preparations alluded to, 

 fig. 6 b; but I cannot positively assert that it had actually 

 separated from that particular part, nor that it was a primary 

 nerve-cell, for the cells in that preparation had not as yet un- 

 dergone any change. In this instance, therefore, we must 

 content ourselves, for the present at least, with the analogy to 

 muscle. 



These fibres, or secondary nerve-cells, differ very much in 

 their appearance from the subsequent nervous fibres, which are 

 furnished with distinct but not dark outlines : thev have a 

 pale, granulated aspect. By progressive development, however, 

 they become converted into the white fibres, and pi. IV, 

 fig. 8, d, represents the transition. The part of the figure 

 to the right hand exhibits the fibre yet in the early condition, 

 pale, granulated, and furnished with a cell-nucleus ; in the 

 portion to the left, it has completely assumed its subsequent 

 form : it has a dark outline, is not granulated, and the one 

 portion passes immediately into the other. The identity be- 

 tween these pale fibres and the subsequent white nervous fibres 

 is thus established. 



In what then does this transformation of the pale granu- 

 lated fibres into the white fibres consist ? Clearly in the 

 development of the white substance ; we may, however, imagine 

 three different modes in which this development may take 

 place. It may take place, lstly. By the white substance form- 

 ing as a sheath (cortex), around each fibre, and in this manner 

 enclosing it. By this mode of explanation the fibre would be 

 identical with the pale band discovered by Remak, which 

 would therefore be the cell-membrane itself. 2dly. The white 

 substance might be regarded as a transformation and thicken- 

 ing of the cell-membrane of those fibres, or secondary nerve- 

 cells. According to this view, the white substance would be 

 the cell-membrane, and Bemak's band the firm contents of the 

 secondary cell. 3dlv. The white substance may be formed as 



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