NERVE 591 



the axis-cylinder process and its collaterals, if it has any, end by 

 breaking up into a brush, a plexus or a feltwprk or basketwork of 

 fibrils. The axons of different nerve-cells vary greatly in length. 

 Some terminate within the grey matter of the brain or spinal cord 

 not far from their origin ; othejrs run in the white tracts of the central 

 nervous system or in the peripheral nerves for half the height of a 

 man. All except the shortest axis-cylinder processes become clothed 

 at a little distance from the cell-body with a protective covering, 

 which continues to invest them (and their collaterals) throughout 

 the. rest of their course, disappearing only when they begin to break 

 up at their terminations. An axis-cylinder process (spoken of simply 

 as the axis-cylinder, when considered apart from the nerve cell) con- 

 stitutes, with its covering, a nerve-fibre. 



- An. ordinary peripheral nerve like the. sciatic is made up of a 

 number of bundles of nerve-fibres. Connective ; tissue surrounds 

 and separates the bundles, and also penetrates in fine septa within 

 them and between the individual fibres, forming a framework for 

 their support, and carrying the bloodvessels and lymphatics. 



The great majority of the nerve-fibres of the sciatic consist of axis- 

 cylinders covered by two sheaths. The axis-cylinders are processes of 

 nerve-cells in the anterior horn of the spinal cord in the case of the 

 motor fibres, and of nerve-cells in the spinal ganglia in the case of 

 the sensory. The axis-cylinder is the essential conducting part of the 

 fibre, for it is present in every nerve-fibre, running from end to end 

 of it without break, and towards the periphery it is alone present. 

 It is probably made up of fine longitudinal fibrils embedded in 

 interstitial substance. Such a fibrillar structure is undoubtedly shown 

 after treatment of the nerve-fibres with certain reagents, although it 

 is not quite certain that it exists preformed in the living fibres. The 

 innermost (Plate V. i, and Fig. 231), and by far the thickest, of the 

 sheaths is the medullary sheath, or white substance of Schwann, 

 which is of fatty nature, and is blackened by osmic acid. It under- 

 goes a kind of coagulation at death, loses its homogeneity, and shows 

 a double contour. This sheath is not continuous, but is broken by 

 constrictions of the outer sheath, called nodes of Ranvier, into 

 numerous segments. The outer sheath, or neurilemma, is a thin, 

 structureless envelope immediately external to the medulla. It 

 invests the nerve-fibre, as the sarcolemma does the muscle-fibre. In 

 each internodal segment immediately under the neurilemma lies a 

 nucleus surrounded by a little protoplasm. Fibres with a medullary 

 sheath such as those described are called medullated fibres. They 

 are by far the most numerous in the cerebro-spinal nerves ; but they 

 are mixed with a few fibres which contain no white substance of 

 Schwann, and are, therefore, called non-medullated (Plate V. i). 

 In these the axis-cylinder is covered only by the neurilemma. In 

 the sympathetic system the non-medullated variety is present in 

 greater abundance than the medullated. In the central nervous 

 system the medullated fibres possess no neuriiemma. 



So far as we know, the only function of nerve-fibres is to 

 conduct impulses from nerve-centres to peripheral organs, 



