144 NERVOUS FIBRES. 



may very probably be overlooked, or possibly be regarded 

 as extraneous substances. But they are in fact the primitive 

 structure of nerve, for the younger the foetus the greater is 

 their relative quantity, and in a pig's foetus of three inches in 

 length, I found them the sole constituent of nerve, none of 

 the fibres furnished with the dark margins, nor any of the 

 cylinders or globules being visible at that period of deve- 

 lopment. The development of nerve, however, does not appear 

 to proceed uniformly in all individuals ; for the dark globules 

 and cylinders were already present in some other pigs' em- 

 bryos, which were scarcely any larger. PI. IV, fig. 6, repre- 

 sents a portion of the ischiatic, and fig. 7, of the brachial 

 nerve of such a foetus. We observe a palish, and very 

 minutely- granulated cord, which, in consequence of certain 

 longitudinal shadings, such as the delineation exhibits, pre- 

 sents the appearance of a coarse fibrous structure. Round or 

 for the most part oval corpuscles, which are immediately recog- 

 nised as cell-nuclei, and which sometimes also contain one or 

 two nucleoli, are generally seen in the course of these shaded 

 parts, throughout the entire thickness of the cord. Sometimes 

 a fibre separates from such a cord, and stands out isolated, as 

 at a in both the figures, and the nuclei are then seen to lie in 

 the course of the fibres. A single fibre presents several nuclei 

 in its course, as was also observed in secondary muscle-cells 

 (see fig. 8, b), but I have never remarked it in the cells of the 

 fourth class, the fibre-cells. Although the (nervous) fibres 

 cannot at this early period be distinctly perceived to be hollow, 

 the wall not being distinguishable microscopically from the 

 contents, yet we shall see that the progress of development 

 renders it highly probable that they are so. If then these 

 (nervous) fibres are so far analogous to the early condition of 

 secondary muscle-cells, that they are hollow, and in various 

 parts of their course contain nuclei, whose form shows them 

 to be ordinary cell-nuclei, it is probable that they are gene- 

 rated in a similar manner to muscle ; that is, that they are 

 formed by the coalescence of primary cells, to which the nuclei, 

 just noticed as present upon the fibres, have pertained; so 

 that thus the nervous fibres would be secondary cells, cor- 

 responding to the secondary muscle-cells, or primitive muscular 

 fasciculi. The actual observation of the primary cells of nerve 



