STRUCTURE OF NERVE-FIBRES. 465 



structure is usually collected in masses, and mingled with 

 the fibrous structure, as in the brain, spinal cord, and the 

 several ganglia ; and these masses constitute what are 

 termed nerve-centres, being the organs in which it is sup- 

 posed that nervous force may be generated, and in which 

 are accomplished all the various reflections and otner 

 modes of disposing of impressions when they are not simply 

 conducted along nerve-fibres. The fibrous nerve- substance, 

 besides entering into the composition of the nervous centres, 

 forms along the nerves, or cords of communication, which 

 connect the various nervous centres, and are distributed in 

 the several parts of the body, for the purpose of conveying 

 nervous force to them, or of transmitting to the nervous 

 centres the impressions made by stimuli. 



Along the nerve-fibres impressions or conditions of ex- 

 citement 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 blood-vessels ramify. A 

 layer of the areolar, or of strong fibrous tissue, also sur- 

 rounds 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 substance, 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 



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