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142 tended sense of the term, including the fibres of the brain and spinal cord: 2d, globules, ganglion-globules, in addition to the ganglia occurring in the brain and spinal cord. Our task is to point out the relation which these two forms of elementary structure bear to the elementary cells.

Of these, there are two different forms: a, the common white nervous fibres ; b, the gray, so-called organic fibres.

a. White nervous fibres. They have the appearance of fibres, which, when examined microscopically, exhibit very dark margins, and these margins are produced by a substance apparently identical with that which gives them their white colour when examined with the unaided eye. Since the cause of this colour does not appear to be situated in the whole fibre generally, but to be confined to its external portion, this latter may be termed the white substance of the nervous fibres. The margin of a fibre generally presents a double outline on both sides, so that it has the appearance of a hollow tube, and the distance between the two outlines, then, denotes the thickness of the white substance. According to the researches of Remak, the white substance of every nervous fibre may be removed by pressure, and an extremely pellucid, pale band, which was previously surrounded by the white substance, then remains, corresponding to that which, previous to the manipulation, seemed to be the contents of the tube. (See R. Remak, Obss. Anat. et Microsc. de Syst. Nerv. Struc., Berol. 1838.)

Two opinions with respect to the nervous fibres may be deduced from the above observations; either this pale band is the proper nervous fibre, and the white substance only a sheath (cortex) around it (this is the view taken by Remak), or the nervous fibre is actually a hollow fibre, the wall of which is formed by the white substance, the contents of which, however, are not fluid, but composed of a tolerably firm substance, namely, the above-mentioned band.

The history of the development of the nervous fibres must