CHAP, i.] THE SPINAL CORD. 853 



to which the nerve is distributed lose the sensibility which they 

 previously possessed. 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 as 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 some- 

 times 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 subse- 

 quently 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 running 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. 



561. Concerning the ganglion on the posterior root, we may 

 say definitely that we have no evidence that it can act as a centre 

 of reflex action; nor have we any evidence 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 seen, these are 

 in reality processes of the nerve cells ; thus the nerve cells in the 

 ganglion appear to be more sensitive to certain poisons than are 

 the nerve fibres of the nerve trunk. But beyond this, our know- 

 ledge concerning the function of the ganglion is almost limited to 

 the fact that it is in some way intimately connected with the 

 nutrition of the nerve. As we have already ( 83) said, when a 

 mixed nerve trunk is divided the peripheral portion degenerates 

 from the point of section downwards towards the periphery. The 

 central portion does not so degenerate, and if the length of nerve 

 removed be not too great, the central portion may grow downwards 

 along the course of the degenerating peripheral portion, and thus 

 regenerate the nerve. This degeneration is observed when the 

 mixed trunk is divided in any part of its course from the periphery 



