CHAPTER VI 

 NERVE FIBRES (CONDUCTING TISSUES) 



SECTION I 

 THE STRUCTURE OF NERVE FIBRES 



ON stimulating the nerve of a nerve-muscle preparation at any part by 

 electrical, thermal, or mechanical means, the stimulus is followed, after 



a very short interval, by a contraction 

 of the muscle. This observation illus- 

 trates the two functions of nerve fibres, 

 irritability and conductivity that is to say, 

 a suitable stimulus can set up changes in 

 any part of the nerve, which are trans- 

 mitted down the nerve without any visible 

 effects occurring in it, and it is not until 

 this nervous change has reached the 

 muscle that a visible effect takes place in 

 the shape of a contraction. In the animal 

 body a direct excitation of the nerve 

 fibre in its course never takes place under 

 normal circumstances. The only function 

 the nerve fibre has to perform is that 

 of conducting impulses from the sense 

 organs at - the periphery to the central 

 nervous system, and efferent impulses 

 from this to the muscles and other of 

 its servants. Hence it is absolutely es- 

 sential that there should be vital continuity 

 along the whole length of the fibre. Dam- 

 age to any part, such as by crushing, heat, 

 or any other injurious condition, infallibly 

 causes a block to the passage of JIM 

 impulse. 



FIG. 100. Diagram of a motor nerve- 

 cell with its nerve-fibre. (After 



hillock ; rf, dendrites ; 

 a.x. axis cylinder; m, medullary 

 sheath ; n.R. node of Ranvirr. 



A nerve fibre is essentially a long process or 

 arm of a nerve-cell (Fig. 100). The cell may 

 either be situated on the surface of the body or, 

 ag in mogfc cagcg in t j ie m -gher animals, may be 

 *ithdWH ^om the surface into a special 

 collection of cells such as the posterior root 

 ganglion, or may be one of the mass of cells and 

 250 



