678 A MANUAL OF PHYSIOLOGY 



which as a rule runs for a considerable distance without altering 

 its calibre, and either gives off no branches (as in the peripheral 

 nerves) or only a comparatively small number of lateral twigs 

 (collaterals). Ultimately the axis-cylinder process and its col- 

 laterals, if it has any, end by breaking up into a brush, a plexus or 

 a feltwork 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 ; others 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-cylindor 

 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 con- 

 sidered apart from the nerve-cell) constitutes, 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 made up of fine longitudinal fibrils embedded in 

 interstitial substance (Fig. 301, p. 748). Such a fibrillar structure 

 is best shown after treatment of the nerve-fibres with certain 

 reagents, although it is certain that it exists preformed in the Jiving 

 fibres. The innermost (Fig. 256), 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 medul- 

 lary 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. In these 

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

 sympathetic system the non-medullated variety is present in 

 greater abundrace than the medullated. In the central nervous 

 system the medullated fibres possess no neurilemma. 



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

 conduct impulses from nerve-centres to peripheral organs, or 

 from peripheral organs to nerve-centres, or from one nerve- 



