156 



PHYSIOLOGY OF NERVE 



[CH. XIV. 



The great discovery that the anterior roots are motor and the 

 posterior sensory is usually attributed to Sir Charles Bell (1811); 

 bub an examination of his writings shows that the deductions he 

 drew were incorrect. It was Magendie (1822) who solved this funda- 

 mental problem, and Herbert Mayo, the first Professor of Physiology 

 at King's College, London, who elucidated similar facts in relation to 

 the cranial nerves which supply motion and sensation in the face 

 region. Magendie found that on section of the anterior roots there 

 resulted paralysis of the muscles supplied by the nerves ; on section 

 of the posterior roots there was loss of sansation. These experiments 

 clearly pointed to the conclusion that the anterior roots contain the 

 efferent (motor) fibres ; and the posterior roots the afferent (sensory) 

 fibres. This conclusion was confirmed by the experiment of stimula- 

 tion. Stimulation of the peripheral end of the cut anterior root caused 

 muscular movement; of the central end, no effect. Stimulation of 



the central end of the cut posterior 

 root caused pain and reflex movements ; 

 of the peripheral end, no effect. 



Recurrent sensibility. One of the 

 statements just made requires a slight 

 modification; namely, excitation of 

 the peripheral end of a divided an- 

 terior root will evoke pain and reflex 

 movements, as well as direct move- 

 ments; that is to say, the anterior 

 root, though composed mainly of 

 motor fibres, contains a few sensory 

 fibres coming from the membranes of 

 the spinal cord, and then running 

 into the posterior root with the rest 

 of the sensory fibres. They often, however, run down the mixed 

 nerve a considerable distance before returning to the posterior roots. 

 The diagram on this page (fig. 160) illustrates the course of one of 

 these recurrent fibres (r) ; the arrows represent the direction in which 

 it conveys impulses. 



Degeneration of roots. The facts in connection with this subject 

 were made out by Waller, and may be best understood by referring 

 to the next diagram (fig. 161). 



A represents a section of the mixed nerve beyond the union of 

 the roots; the whole nerve beyond the section degenerates, and is 

 consequently shaded black in the figure. 



B represents the result of section of the anterior root ; only the 

 anterior root-fibres degenerate; the sensory fibres of the posterior 

 root remain intact. The small medullated nerve-fibres (not shown in 

 the diagram) also degenerate as far as the ganglion cells of the 



Spinal Nerve 



FIG. 160. Diagram to illustrate recurrent 

 sensibility. 



