THE NERVOUS SYSTEM. k87 



of numerous dark-bordered fibres. In the nerves of the 



spleen, in the Calf, in like manner, numerous nucleated fibres 

 are met with, though in the terminations (vide ' Cyclopaedia of 

 Anatomy/ III, p. 795, figs. 539 and 540), which, probably, 

 afterwards become nerve-fibres. In youny animals, conse- 

 quently, ive must not look for a decision of the question; whilst 

 in older ones, it is quite otherwise. In them, a nucleated fibre 

 can only be regarded as nerve when it can be traced into a dark- 

 bordered fibre, or to a true process of a nerve-cell ; and this, as 

 we have seen, is not the case in those of the sympathetic 

 system. It may, however, be remarked, that " fibres of 

 Remak" also occur in the ganglia of the main sympathetic 

 trunk, but that they do not, for the most part, extend to any 

 distance beyond them, so that usually but few are contained 

 in the trunk of the nerve itself.] 



§ 126. 



Development of the elements of the Nervous System. — The 

 nerve-cells, wherever they may occur, are nothing else than 

 transformations of the so-called embryonic-cells; some of which 

 simply enlarge, whilst others throw out 

 a varying number of processes, and are, Flg * 164, 



at all events in part, connected with 

 nerve-fibres. 



Many nerve-cells also appear, at a 

 subsequent period, to increase by division; 

 at all events, I do not know how other- 

 wise to explain the frequent occurrence 

 of two nuclei in the nerve-cells of young 

 animals, especially in the ganglia ; and 

 the cells connected by communicating 

 filaments, which have been noticed by 

 various observers. 



The peripheral nerve-fibres all originate on the spot, but 

 their subsequent development proceeds in such a way that the 

 central extremities always precede the peripheral. With the 



Fig. 164. Nerve-cell from a spinal ganglion of a sixteen-weeks' human embryo: 

 a, nucleus in the pale process of the cell ; 2, self-developing nerve-tubes from the 

 brain of a two-months' human embryo; 3, cells from the grey cerebral sub&tance of 

 the same embryo. 



