THE NERVOUS SYSTEM. 



427 



ncrve-cclls, wherever they may occur, arc nothing else than transforma- 

 tions of the so-called embryonic-cells; some of which simply enlarge, 

 whilst others throw out a varying number of processes, and are, 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 otherwise to explain the fre- 

 quent 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. 



Fig. 164. 



The peripheral nerve-jihres all originate on the spot, but their subse- 

 quent development proceeds in such a way that the central extremities 

 always precede the peripheral. With the exception of the extremi- 

 ties of the nerves, they are developed from fusiform nucleated cells, 

 which are nothing else than modifications of the primordial formative 

 cells of the embryo, and are conjoined into pale, flattened, elongated, 

 nucleated tubules or fibres 0-001-0-003 of a line, broad. Now, at first 

 the nerves consist only of fibres of this kind, and of the rudiment of 

 the neurilemma, being gray or dull white, like the sympathetic filaments ; 

 subsequently, in the human embryo at the fourth or fifth month, they 

 always assume a whiter color, and the proper white or medullary sub- 

 stance continues to be more and more developed in them. Of the three 

 possible modes of development of this substance propounded by Schwann, 

 one only, in the present state of things, can come into question, that 

 namely, as to whether the medullary sheath is a structure deposited 

 between the membrane and the contents of the embryonic nucleated 

 fibres; in which case the contents of the latter would become the axis- 

 fibre. But besides this, the medullary sheath may originate in what did 



Fig. IGt. — 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 frotn the gray cerebral substance of the same embryo. 



Fig. 165. — 1, two nerve-fibres from the ischiatic nerve of a sixteen- weeks' embryo; 2, 

 nerve-tubes from a newly-littered Rabbit; a, their sheath ; 6, nucleus; c, medullary sheath; 

 3, nerve-fibre from the tail of the Tadpole ; a, b, c, as before ; at d, the fibre retains its 

 embryonic character; the dark-bordered fibre shows a division. 



