CH. XV.] 



IIECURREXT SENSIBILITY 



169 



Posterior Root 



process grows outwards to the periphery. In the adult mammal 

 (not in fishes) the two processes coalesce in the first part of their 

 course, forming a T-shaped junction. 



The first experiments on the functions of the spinal nerve-roots 

 were performed in this country by Sir Charles Bell (1811), and in 

 France by Magendie (1822). These observers 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 

 sensation. 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 stimulation. 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 probably from the mem- 

 branes 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. 188) 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 (1850), and may be best understood by 

 referring to the next diagram (fig. 189). 



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

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

 shaded black. 



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 



FIG. 188. Diagram to illustrate recurrent 

 sensibility. 



