THE NERVE-IMPULSE OR PROPAGATED DISTURBANCE 781 



laterals). Ultimately the axis-cylinder process and its collaterals, 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-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) constitutes, 

 with its covering, a nerve-fibre. 



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. 329, p. 851). 

 Such a fibrillar structure is best shown after treatment of the nerve- 

 fibres with certain reagents, although it is certain that it exists pre- 

 formed in the living fibres. 



SECTION I. THE NERVE- IMPULSE OR PROPAGATED DISTURBANCE: 

 ITS INITIATION AND CONDUCTION. 



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-centre to another. 

 In the normal body these impulses never, or only very rarely, 

 originate in the course of the nerve- fibres; they are set up either 

 at their peripheral or at their central endings. By artificial stimu- 

 lation, however, a nerve-impulse may be started at any part of a 

 fibre, just as a telegram may be dispatched by tapping any part of 

 a telegraph wire, although it is usually sent from one fixed station 

 to another. 



Nature of the Nerve- Impulse. What the nerve-impulse actually 

 consists in we do not know. All we know is that a change or dis 

 turbance of some kind, of which the most evident token is an 

 electrical change, passes over the nerve with a measurable velocity, 

 and gives tidings of itself, if it is travelling along efferent fibres 

 that is, out from the central nervous system by the contraction 

 or inhibition of muscle or by secretion; if it is travelling along 

 afferent fibres that is, up to the central nervous system by sensa- 

 tion, or by reflex muscular or glandular effects. 



Whether the wave which passes along the nerve is a wave of 

 chemical change (such, to take a very crude example, as runs 

 along a train of gunpowder when it is fired at one end), or a wave 

 of mechanical change, a peculiar and most delicate molecular 

 shiver, if we may so phrase it, or a shear in a definite direction along 

 the colloidal substance of the axis-cylinder (Sutherland), there is 

 no absolutely definite experimental evidence to decide. An 



