730 



STRUCTURE OF NERVE-SUBSTANCE. 



brown colour ; and thus impart to collections of ganglionic-cells in 

 the warm-blooded Vertebrata that peculiar hue, which causes it to 



be known as the Cineritious 

 Fig. 390. or Grey matter ; these, how- 



ever, are commonly absent 

 among the lower Animals. — 

 Each of the Nerve-Tubes, on 

 the other hand, of which the 

 trunks are composed, con- 

 sists, in its most completely- 

 developed form, of a delicate 

 membranous sheath, within 

 which is a hollow cylinder 

 of a material known as the 

 ' white substance of Schwann, ' 

 whose outer and inner boun- 

 daries are marked out by two 

 distinct lines, giving to each 

 margin of the nerve - tube 

 what is described as a 'double 

 contour.' The centre or axis 

 of the tube is occupied by a 

 transparent substance which 

 is known as the 'Axis- 

 Cylinder :' and there is reason 

 to believe that this last, which 

 is a Protoplasmic substance, 

 is the essential component of 

 the Nerve-fibre, and that the hollow cylinder which surrounds 

 it, and which is composed of a combination of Fat and Albu- 

 minous matter, serves, like the tubular sheath, for its complete 

 isolation. The contents of the membranous envelope are very 

 soft, yielding to slight pressure ; and they are so quickly altered 

 by the contact of water or of any liquids which are foreign to 

 their nature, that their characters can only be properly judged-of 

 when they are quite fresh. — Besides the proper Tubular Fibres, 

 however, there are others, known as ' Gelatinous, ' which are 

 considerably smaller than the preceding, and do not exhibit any 

 differentiation of parts (Fig. 391). They are flattened, soft, and 

 homogeneous in their appearance, and contain numerous Nuclear 

 particles, which are brought into view by Acetic acid. They can 

 sometimes be seen to be continuous with the axis-cylinders of the 

 ordinary Fibres, and also with the radiating prolongations of the 

 Ganglion -cells ; so that their Nervous character, which has been 

 questioned by some anatomists, seems established beyond doubt. 



572 ; The ultimate distribution of the Nerve-fibres is a subject 

 on which there has been great divergence of opinion, and which 

 can only be successfully investigated by observers of great experi- 



Ganglion-cells and Nerve-fibres, from 

 a Ganglion of Lamprey. 



