SECT. 1 10.] NERVOUS SYSTEM. 21 7 



The contents of the nerve-cells are a soft, but viscid, elastic sub- 

 stance, which, apart from the cell-nucleus, consists of two parts : 

 firstly, of a clear, homogeneous, slightly yellowish, or colourless 

 matrix, which determines the physical properties of the contents of 

 the nerve-cells, and is, for the most part, a protein compound; 

 and, secondly, of fine granules of various kinds. In the colourless 

 nerve-cells, the granules are distributed through the whole contents, 

 and imbedded in the tenacious matrix in form of uniformly large, 

 rounded, mostly very fine and pale, more rarely larger and dark, 

 particles ; whilst in the coloured cells, instead of these, there are 

 found particles of a more or less yellowish brown, or black colour. 

 These latter are mostly large, and usually lie close together at one 

 part of the cell, in the neighbourhood of the nucleus; at other 

 times, they almost completely fill the cell, and give it the appear- 

 ance of a brown or black pigment-cell. In the midst of these 

 contents the cell-nucleus is embedded, and is, for the most part, 

 distinctly visible as a spherical vesicle, with well-defined Avails, per- 

 fectly clear fluid contents, and one, or more rarely several, dark, 

 large nucleoli, which occasionally contain a cavity in their interior. 



The nerve-cells are very various in magnitude, being, like the 

 fibres, of large, small, and medium size. The extremes for the 

 cells are 000?.'" to o - oo3'", and cro5'" to o , o6 / " in diameter. The 

 nuclei, which mostly correspond to the cells, measure from o , ooi5'" 

 to o'oo8'"; the nucleoli, from o - ooo5'" to o - oo3". The nerve-cells 

 are further distinguished : 1. into thin-walled and thick-walled, of 

 which the former are almost all found in the brain and spinal cord ; 

 and 2. into independent cells and cells icith pale processes, which are 

 either single, or in pairs, or several (uni-, bi-, multipolar cells), 

 and ramify frequently ; these processes are, in many places, con- 

 tinued into dark-bordered nerve-fibres, and are even to be regarded 

 as non-medullated nerve-fibres. 



Besides the nerve-cells, there exist in the grey substance of the 

 higher central organs, as constant constituents, a finely granular 



le substance which bears the greatest resemblance to the contents 

 of the cells, and large local collections of free cell-nuclei. The 

 retina, and, according to Wagner and Robin, the ganglia of pla- 

 giostomatous fishes, contain similar elements. 



It is still undetermined whether or not the nerve-cells of the great central 

 organs possess membranes. Stannius could not find them in the lamprey, 

 and R. Wagm r states the same to be true of the nerve-cells in the electric 

 lobes of the torpedo. I believe I have seen a membrane in the large stellate 

 corpuscles in the human spinal cord and cerebellum, and occasionally, also, 

 in others ; still I willingly admit, that in all the smaller cells, and the pro- 

 cesses of the central cells in general, no covering can be perceived. This 



