GENERAL STRUCTURE AND FUNCTIONS 



black color; and by this mode of preparation nervous fibres of extreme 

 delicacy have been traced among surrounding tissues, where they would 

 otherwise escape observation. 



In its physiological properties, the axis cylinder is undoubtedly the 

 most essential element of the nerve fibre, since it is the only one univer- 

 sally present, and always extending throughout the whole length of a 

 fibre from its origin to its termination. Its albuminous nature also 

 distinguishes it from other parts of the nerve fibre, and indicates the 

 relative importance of its function. It is probably through the axis 

 cylinder that the passage of the nerve current takes place, and in its 

 substance that the principal changes accompanying this action are 

 effected. 



Non-Medullated Nerve Fibres. Beside the nerve fibres constituted, 

 as above, by an axis cylinder, surrounded by a medullary layer, with or 

 without an external tubular membrane, there are others which consist 

 of the axis cylinder destitute of any medullary layer, and which conse- 

 quently do not exhibit the appearance of a double contour. These are 

 called " non-medullated nerve fibres." They are found, in man, only in 

 certain parts of the sympathetic nerves, in the terminal nervous expan- 

 sions of the muscles and organs of sense, and in the nervous centres in 

 the immediate vicinity of the cells of the gray substance. In the sym- 

 pathetic nerves, they are, for the most part, mingled with a considerable 

 proportion of inedullated fibres, though some of the sympathetic branches 

 distributed to the intestine and the spleen, according to Schultze, are 

 composed of non-medullated fibres exclusively. The branches of the 

 olfactory nerve, distributed to the nasal mucous membrane, also consist 

 altogether of fibres of this kind. Such nervous branches have not the 

 white, opaque aspect belonging to other nerves, but are grayish-looking 

 and semi-transparent in appearance ; a peculiarity which is evidently due 

 to the absence of the myeline or medullary layer. 



The same nerve fibre may be inedullated for the greater part of its 

 course, and become destitute of medulla at its termination, as is the rule 

 with the cerebro-spinal nerves generally ; or fibres may originate in the 

 gray substance as non-medullated axis cylinders, and become invested, 

 after a short distance, with a distinct medullary layer. The non-medul- 

 lated nerve fibres are not, therefore, regarded as essentially different 

 from the others, but only as presenting a less complicated form of struc- 

 ture. 



Course and Mutual Relation of the Nerve Fibres. In the white sub- 

 stance of the brain and spinal cord, the nerve-fibres form continuous 

 tracts, of larger or smaller size, lying in contact with each other, and 

 not mingled with any considerable proportion of other tissue. But on 

 passing out of the bony cavities toward the exterior, they become col- 

 lected into small bundles, each of which is invested with a thin pro- 

 longation of connective tissue, derived from the dura mater and 

 periosteum ; these bundles are associated into larger ones which are 

 held together by a denser layer of the same connective tissue ; and 



