STRUCTURE OF NERVE- FIB RES. 



369 



FIG. 133. 

 B c 



Along the nerve-fibres impressions or conditions of excite- 

 ment are simply conducted : in the nervous centres they may 

 be made to deviate from their direct course, and be variously 

 diffused, reflected, or otherwise disposed of. 



Nerves are constructed of minute fibres or tubules full of 

 nervous matter, arranged in parallel or interlacing bundles, 

 which bundles are connected by intervening connective tissue, 

 in which their principal bloodvessels ramify. A layer of the 

 areolar, or of strong fibrous tissue, also surrounds the whole 

 nerve, and forms a sheath or neurilemma for it. In most 

 nerves, two kinds of fibres are mingled ; those of one kind 

 being most numerous in, and charac- 

 teristic of, nerves of the cerebro- 

 spinal system ; those of the other, 

 most numerous in nerves of the 

 sympathetic system. 



The fibres of the first kind appear 

 to consist of tubules of a pellucid 

 simple membrane, within which is 

 contained the proper nerve sub- 

 stance, consisting of transparent oil- 

 like, and apparently homogeneous 

 material, which gives to each fibre 

 the appearance of a fine glass tube 

 filled with a clear transparent fluid 

 (Fig. 133, A). This simplicity of 

 composition is, however, only ap- 

 parent in the fibres of a perfectly 

 fresh nerve ; for shortly after death, 

 they undergo changes which make 

 it probable that their contents are 

 composed of two different materials. 

 The internal or central part, occu- 

 pying the axis of the tube, becomes 

 grayish, while the outer, or cortical 

 portion, becomes opaque and dimly 

 granular or grumous, as if from a 

 kind of coagulation. At the same 

 time, the fine outline of the pre- 

 viously transparent cylindrical tube 

 is exchanged for a dark double con- 

 tour (Fig. 133, B), the outer line 

 being formed by the sheath of the 

 fibre, the inner by the margin of 

 curdled or coagulated medullary substance. The granular 



Primitive nerve-tubules. A. 

 A perfectly fresh tubule with 

 a single dark outline. B. A 

 tubule or fibre with a double 

 contour from commencing 

 post-mortem change, c. The 

 changes further advanced, 

 producing a varicose or beaded 

 appearance. D. A tubule or 

 fibre, the central part of which, 

 in consequence of still further 

 changes, has accumulated in 

 separate portions within the 

 sheath (after Wagner). 



