OF THE NERVOUS SYSTEM. 



401 



Fig. 134. 



locality ; some of the fibres in a single bundle being 10, 15 or 18 micro- 

 millimetres in diameter, while others are not more than 2.5 mmm. Their 

 average size also varies in different parts of the nervous system. The 

 larger fibres are found in the peripheral trunks and branches of the 

 nerves, where they have an average diameter of 12.5 mmm. ; in the 

 white substance of the brain and spinal cord their average diameter is 

 5 mmm., and in the gray substance it is reduced to 2 mmm. Two por- 

 tions of the nervous system, both of which contain nerve fibres, are 

 often distinguished from each other by the relative numbers of their 

 larger and smaller fibres. Thus in the cutaneous nerves of man, accord- 

 ing to Bidder, Yolkmann, and Kolliker, the larger and smaller fibres are 

 present in about equal quantity, while in the muscular nerves the larger 

 fibres are three times as abundant as the smaller. In the nerves of bony 

 tissue the proportion of small fibres is double that of the large ones, 

 and in the gray substance of the cerebral hemispheres there are none 

 larger than 6 or 7 mmm. in diameter. The nerve fibres belonging to the 

 same bundle or tract may even become increased or diminished in diameter 

 in different parts of their course ; as Kolliker has shown that the fibres 

 of the posterior roots of the spinal nerves, in passing through the cord 

 from the exterior to the gray 

 substance, are reduced in their 

 average diameter from 10 to 5 

 mmm. ; and those of the white 

 substance, of the cerebral hemi- 

 spheres, on entering the gray 

 matter of the convolutions, are 

 reduced from 5 mmm. to 2 

 mmm. in diameter. 



The structure of the nerve- 

 fibre, in its most complete 

 form, presents three distinct 

 elements, namely : an external 

 tubular sheath, an intermediate 

 medullary layer, and a central 

 axis cylinder. 



The Tubular Sheath The 



exterior of the nerve fibre is 

 composed of a colorless, trans- 

 parent tubular membrane, 

 which closely invests the re- 

 maining portions and is seen 

 with some difficulty in the 

 natural condition of the fibre, owing to its extreme thinness and 

 delicacy. It may often, however, be distinguished at certain points 

 where the nervous fibre is accidentally compressed or indented, as at c, 

 Fig. 134; or it may be brought into view for considerable distances 

 according to the method of Kolliker, by treating the fibres with a cold 



NBRVB FIBRES from the Sciatic Nerve. At 

 a, the torn extremity of a nerve fibre with the axis 

 cylinder (b) protruding from it. At c, the medul- 

 lary layer is nearly separated by accidental com- 

 pression, but the axis cylinder passes across the 

 injured portion. The outline of the tubular mem- 

 brane is also seen at c on the outside of the remain- 

 ing portions of the fibre. 



