ON SOME FEATURES OF THE SPINAL NERVES. 527 



We shall further see in detail later on that our consciousness may be 

 affected in many different ways by afferent impulses ; we must distinguish 

 not only sensory from other afferent impulses, but also different kinds of 

 sensory impulses from each other. Certain afferent nerves are spoken of as 

 nerves of special sense, and the nature of the afferent impulses passing along 

 these special nerves, together with the modifications of consciousness caused 

 by arrival of these impulses at the central nervous system, constitute by them- 

 selves a complex and difficult branch of study. In some of the problems 

 connected with the central nervous system we shall have to appeal to the 

 results of a study of these special senses ; but, on the other hand, a knowl- 

 edge of the central nervous system is necessary to a proper understanding 

 of the special senses ; and on the whole it will be more convenient to study 

 the former before the latter. 



473. The proof that the afferent and efferent fibres which are both 

 present in the trunk of a spinal nerve are parted at the roots, the efferent 

 fibres running exclusively in 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 sensibil- 

 ity. During the section of the root, or when the proximal stump connected 

 with the spinal cord is stimulated, no sensory effects are produced. When 

 the distal stump is stimulated the muscles supplied 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 possessed. During the section of 

 the root, and when the approximal stump is stimulated, the 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, an account 

 of the so-called " recurrent sensibility " which is witnessed in conscious mam- 

 mals under certain circumstances. It sometimes happens that when the 

 distal stump of the divided anterior root is stimulated, signs of pain are wit- 

 nessed. 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 running a short distance down the 

 mixed trunk, turn back and run upward in the anterior root (being distrib- 

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

 the recurrent sensibility. 



474. Concerning the ganglion on the posterior root, we may say de- 

 finitely 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 



