PKOPAGATION ALONG NERVE FIBRES 



255 



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 doivn 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 

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

 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 there- 

 fore not so much on the structure of the nerve fibre itself as on the 

 connections of the fibre. We can show this experimentally by graft- 

 ing 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 determined 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 the skin 

 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. 



FIG. 104. 



Kiihne's gracilis 



experiment. 



