THE NERVOUS SYSTEM. 



357 



externally by a delicate, structureless membrane^ which in the cells of 

 the ganglia (ganglion-cells, -globules, -corpuscles), may be demonstrated 

 easily, but Avith great difficulty in those of the central organs ; the 



Fi?. 140. 



a... 



application of reagents, however, will suffice to show, pretty distinctly, 

 that the membrane exists around the larger cells, even in these situa- 

 tions, whilst in the smallest, just as in the finest nerve-fibres, no mem- 

 brane has yet been observed, although one probably exists. The contents 

 of the nerve-cells are a soft, but tenacious, elastic substance, which be- 

 sides the nucleus consists of two elements ; firstly, of a clear, homoge- 

 neous, light-yellowish, or colorless matrix, upon which the physical 

 properties of the contents depend, and which is a protein-compound ; 

 and, secondly, of minute granules of different kinds. In the colorless nerve- 

 cells these present the form of uniform, roundish, for the most part, 

 minute and pale, more rarely, darker and larger corpuscles dispersed 

 throughout the entire contents of the cells, and imbedded in the tena- 

 cious matrix ; whilst in the colored cells, instead of these granules, more 

 or less yellowish, brown or blackish corpuscles occur. The latter are 

 most usually of a larger size, and are placed, closely aggregated, in a 

 mass near the nucleus ; in other instances, they nearly fill the entire 

 cell, giving it the aspect, in all respects, of a brown or blackish pigment- 

 cell. In the midst of these contents lies the nucleus, for the most part 

 as a very clear, spherical vesicle with distinct walls, perfectly transpa- 



FiG. 140. — Nerve cells, from the acoustic nerve, magnified 350 diameters: 1, nerve-cells 

 with tiie origin of a fibre, from the anastomosis between the facia! and auditory nerves, in 

 the meatus audit, int. of the Ox ; a, membrane of the cell ; b, contents ; c, pigment ; d, nucleus ; 

 e, continuation of the sheath upon the nerve-fibre ;/, nerve-fibre : 2, two nerve-cells with 

 fibres, from the w. ampull. infer, of the Ox ; a, sheath with nuclei ; 6, membrane of the cell ; 

 c, nucleus ; d, the origin of a fibre with nucleated sheath : 3, isolated contents of a nerve- 

 cell, with nucleus and two nucleoli. For these drawings I am indebted to Dr. Corii. 



