The brain as an organ of mind . Fio. 3.—Human Nerve Fibresof different sizes (Kolliker). a, a, a. Healthy fibres, thelargest of which is dark-bor-dered. b, b. Fibres altered byexposure. Magnified 350 dia-meters. 32 THE STRUCTURE OF lated nerve fibres speedily undergo chanoe. They then notunfrequently assume an irregular or varicose appearance—•. PiO. 4.—Small Branch of a Muscular Nerve of the Frog, near its Termination,showing divisions of the Fibres. Magnified 350 diameters (KoUiker). <f, into two ;h, into three. principally owing to changes in the white medullarysheath (fig. 3, h, h). The
The brain as an organ of mind . Fio. 3.—Human Nerve Fibresof different sizes (Kolliker). a, a, a. Healthy fibres, thelargest of which is dark-bor-dered. b, b. Fibres altered byexposure. Magnified 350 dia-meters. 32 THE STRUCTURE OF lated nerve fibres speedily undergo chanoe. They then notunfrequently assume an irregular or varicose appearance—•. PiO. 4.—Small Branch of a Muscular Nerve of the Frog, near its Termination,showing divisions of the Fibres. Magnified 350 diameters (KoUiker). <f, into two ;h, into three. principally owing to changes in the white medullarysheath (fig. 3, h, h). The use of this white investin<]C substance is not known. Chap. II.] A NERVOUS SYSTEM. 33 It is absent from the peripheral extremities of the nerves,and it is absent also from their central extremities, at thepoints where the fibres approach or depart from the nervecells. Both it and the membranous investing sheath havebeen of late ascertained to be regularly interrupted atcomparatively short distances, so that such nerve fibreshave the appearance of being constricted in these situa-tions (Ranvier). Nearly all visceral nerves, as well as the fibres of theolfactory and some others, do not possess this medullarysheath, to which the dead white colour of the great ma-jority of nerve fibres is due. They are, therefore, semi-translucent or g
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