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 func- 

 tion 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 tms 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. Damage 

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

 any other injurious condition, infallibly 

 causes a block to the passage of an im- 

 pulse. 



A nerve fibre is essentially a long pro- 

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



FIG. 100. Diagram of a motor nerve- ce H may e i t h er be situated on the surface of 

 cell with its nerve fibre. (After . , , . ' -11-1 



BARKER.) the body or, as in most cases in the higher 



a.h, axon hillock ; d, dendrites ; animals, may be withdrawn from the 



a.x, axis cylinder; m, medullary < , i n , r n i_ 



sheath; n.R, node of Ranvier. surtace into a special collection 01 cells such 



250 



