404 SPECIAL HISTOLOGY. 



yet been observed, although one probably exists. The contents 

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

 which besides the nucleus consists of two elements; firstly, of 

 a clear, homogeneous, light-yellowish, or colourless 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 colourless 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 cell, and 

 imbedded in the tenacious matrix ; whilst in the coloured 

 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 transparent contents, and one, or more rarely 

 several, large opaque nucleoli, which occasionally exhibit a cavity. 

 The size of the nerve-cells varies very much; like the fibres, 

 they occur as large, small, and middle-sized. The extreme 

 dimensions of the cells are 0002 — 0-003'" and 0-05 — 0-06'". 

 The nuclei, which for the most part are in proportion to the 

 cells, measure from 0-0015 — 0-008'"; the nucleoli 0-0005 — 

 0-003'". The nerve-cells, moreover, are distinguished accord- 

 ing as they are: 1. thin or thick-trailed, of which the former 

 are found almost wholly in the spinal-cord and brain ; and 2. as 

 they are independent cells, or are furnished ivith pale processes, 

 of which they may have one, two, or several (uni-, bi-, multi- 

 polar cells), and which are frequently ramified, and the former, 

 in many situations, continuous with dark-bordered nerve-fibres, 

 and even having the nature of non-medullated nerve-fibres. 

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

 of the higher central organs, as constant constituents, a finely 

 granular pale substance, which has the greatest resemblance to 

 the contents of the cells, and besides this, in places, large 

 accumulations of free cell-nuclei. Similar elements are con- 

 tained in the retina, and according to Wagner and Robin in 

 the ganglia of the Plagiostomata. 



