CHAPTER VI 

 NERVE FIBRES (CONDUCTING TISSUES) 



SECT] ON I 

 THE STRUCTURE OF NERVE FIBRES 



ON stimulating the nerve 

 part by electrical, thermal, 



FIG. 101. Diagram of a motor nerve- 

 cell with|its nerve fibre. (After 

 BARKER.) 



.&, axon hillock; d, deudrites; 

 a.x, axis cylinder; m, medullary 

 sheath ; n.R, node of Ranvier. 



of a nerve- muscle preparation at any 

 or mechanical means, the stimulus is 

 followed, after a very short interval, by 

 a contraction of the muscle. This obser- 

 vation illustrates the two functions of 

 nerve fibres, irritability and conducti- 

 vity that is to say, a suitable stimulus 

 can set up changes in any part of the 

 nerve, which are transmitted down the 

 nerve without any visible effects occur- 

 ring 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 nor- 

 mal 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 

 essential 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 in- 

 jurious condition, infallibly causes a 

 block to the passage of an impulse. 



A nerve fibre is essentially a long 

 process or arm of a nerve-cell (Fig. 101). 

 The cell may either be situated on the 

 surface of the body or, as in most cases 

 in the higher animals, may be withdrawn 

 279 



