678 SPINAL NERVES. [BOOK m. 



the ventral or anterior root and the afferent fibres exclusively 

 in the dorsal or posterior root, is as follows. 



When the anterior root is divided, the muscles supplied by 

 the nerve cease to be thrown into contractions either by the 

 will, or by reflex action, while the structures to which the 

 nerve is distributed retain their sensibility. During the sec- 

 tion of the root, or when the proximal stump, that connected 

 with the spinal cord, is stimulated, no sensory effects are pro- 

 duced. When the distal stump is stimulated, the muscles sup- 

 plied by the nerve are thrown into contractions. When the 

 posterior root is divided, the muscles supplied by the nerve 

 continue to be thrown into action by an exercise of the will or 

 as part of a reflex action, but the structures to which the nerve 

 is distributed lose the sensibility which they previously pos- 

 sessed. During the section of the root, and when the proximal 

 stump is stimulated, sensory effects are produced. When the 

 distal stump is stimulated no movements are called forth. 

 These facts demonstrate that sensory impulses pass exclusively 

 by the posterior root from the peripheral to the central organs, 

 and that motor impulses pass exclusively by the anterior root 

 from the central to the peripheral organs ; and so far as our 

 knowledge goes the same holds good not only for sensory and 

 motor but also for afferent and efferent impulses. 



An exception must be made to the above general statement, 

 on account of the so-called "recurrent sensibility" which is 

 witnessed in conscious mammals, under certain circumstances. 

 It sometimes happens that when the distal stump of the divided 

 anterior root is stimulated, signs of pain are witnessed. These 

 are not caused by the concurrent muscular contractions or 

 cramp which the stimulation occasions, for they persist after 

 the whole trunk of the nerve has been divided some little way 

 below the union of the roots above the origins of the muscular 

 branches, so that no contractions take place. They disappear 

 when the posterior root is subsequently divided, and they are 

 not seen if the mixed nerve-trunk be divided close to the union 

 of the roots. The phenomena are probably due to the fact, 

 that bundles of sensory fibres of the posterior root after run- 

 ning a short distance down the mixed trunk turn back and run 

 upwards in the anterior root, (being distributed probably to 

 the pia mater,) and by this recurrent course give rise to the 

 recurrent sensibility. 



451. Concerning the ganglion on the posterior root, we 

 have already said that we have no evidence either that it can 

 act as a centre of reflex action, or that it can spontaneously 

 give origin to efferent impulses and thus act as an automatic 

 centre, as can the central nervous system itself. The bodies 

 of the nerve-cells behave somewhat differently from the axis- 

 cylinders at some distance from the cells, though, as we have 



