PROPAGATION ALONG NERVE FIBRES m 



the whole muscle contracts. If the portion of the muscle which is free from 

 nerve fibres be stimulated in the same way, the contraction is limited to 

 the fibres directly stimulated, showing that in the first case the stimulus 

 excited nerve fibres which transmitted the impulse up the nerve to the point 

 of division and then down again to the other half of the muscle. 



Since nerves have this power of conduction in both directions, it might 

 be thought that a single set of nerve fibres might very well subserve both 

 afferent and efferent functions, at one time conducting 

 sensory impulses from periphery to cord, at another time 

 motor impulses from cord to muscles. But this is not the 

 case. As a matter of fact we find in the body a marked 

 differentiation of function between various nerve fibres. 

 Thus Bell and Majendie showed that the spinal roots 

 might be divided into afferent and efferent, the anterior 

 roots carrying only impulses from spinal cord to periphery^ 

 while the posterior roots carried impulses from periphery 

 to central nervous system. The law known by the name 

 of these observers states indeed that a nerve fibre cannot 

 be both motor and sensory. We may find both kinds of ... FlG ; 104 - 

 fibres joined together into a single nerve- trunk, but the experiment! 15 

 fibres in each case are isolated and conduct impulses only 

 in one or other direction. Under normal conditions the afferent fibres are 

 excited only at their endings on the surface of the body, while the efferent 

 fibres are excited only at their origin from the spinal cord. The difference 

 in the function of different nerve fibres depends therefore not so much 

 on the structure of the nerve fibre itself as on the connections of the fibre. 

 We can show this experimentally by grafting one set of nerve fibres on 

 to another. If the cervical sympathetic be united to the lingual nerve, 

 stimulation of the sympathetic, instead of causing, as usual, constriction 

 ( of the vessels of the head and neck, will cause dilatation of the vessels of the 

 tongue and secretion of watery saliva. In the same way the finer functional 

 differences between the various forms of sensory nerves seem to be deter- 

 mined by their connections within the central nervous system. Stimulation 

 of the optic nerve by any means whatsoever .evokes a sensation of light. 

 One and the same stimulus applied to different nerves will evoke different 

 sensations, e.g. a tuning-fork applied to therskin will give a sensation of 

 vibration, to the ear a sensation of sound. We shall have occasion to return 

 to this question of the restricted function of nerve fibres when we deal with 

 Miiller's ' law of specific irritability ' in the chapter on Sensations. 



