STRUCTURE OF NERVE-FIBRES. 467 



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 other 

 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 alone 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-fibro-cellular 

 tissue, in which their principal blood-vessels ramify. A 

 layer of the same, 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 characteristic 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. 126) A). This simplicity of 

 composition is, however, only apparent in the fibres of 

 a perfectly fresh nerve ; for, shortly after death, they 



