. THE NERVOUS SYSTEM IN GENERAL. 743 



Properties and Functions of the Nervous System. 



It would require a long chapter to do justice to this subject, and we could 

 not venture on it here without going beyond our domain. We will, however, 

 offer some remarks on those notions connected with the properties and functions 

 of the nervous system, which are strictly necessary for the comprehension of the 

 anatomical facts to be hereafter dealt with. 

 And first as to the properties of the nerves. 



"We will suppose the spinal canal to be opened in the lumbar region, and the 

 cord laid bare in a living animal. If we cut across the inferior roots of one of 

 the spinal nerves, and compress with a pair of forceps one or more of these roots 

 by the end remaining attached to the cord, nothing results to denote that this 

 irritation has had any influence on the organism. But if, instead of operating 

 on the central or attached end of these divided roots, we excite the peripheral 

 end which is continued by the trunk of the nerve, contraction of the muscles of 

 the limb which receives the fibres coming from the irritated roots is produced. 



The muscular tissue comports itself as if the irritation were directly applied 

 to it ; so that the nerve has served as the medium of communication. It has 

 received the stimulus, it has been excited by it, and it has conducted this to the 

 muscles to which the nerve is distributed. This double reaction produced by 

 the nerve-tubes is their special attribute, their essential property. With Vulpian, 

 we might designate it collectively by the term neurility ,- but it is necessary to 

 distinguish the two modes it affects, by naming the property of being impressed 

 by stimuli as the excitahility of the nerve, and nervous conductibility its aptitude 

 to convey the excitations which have impressed it. 



The same experiment may be repeated on the upper roots. It is then per- 

 ceived that the pinching, which produces no effect at the peripheral extremity, 

 causes pain when applied to the central end. The animal testifies immediately, 

 by cries and movements, that it feels the touch of the forceps. But, as will be 

 mentioned in a moment, the impression resulting from this touch has only been 

 perceived by the brain ; it has therefore been conducted to the spinal cord by the 

 stimulated nerve-fibres, and then to the brain by the fibres of this medullary axis. 

 In putting to one side, for the moment, the part played by the latter in the 

 phenomenon now analyzed, it will be seen that the superior fibres of the spinal 

 nerves enjoy the same attributes as the inferior ; neurility is their appanage, and 

 this property is apparent in its two qualities — excitahility and conductiMlity . 

 Only here the latter property is exercised in a centripetal sense ; while in the first 

 instance it acted in a centrifugal sense. But it must not be assumed that these 

 two conductibilities are essentially distinct. The physiological differences by 

 which they appear to be distinguished, seem to belong to the difference in the 

 relations of the nerve-fibres with the organs to which they are distributed. In 

 one case (that of the centrifugal nprves), the organs of reaction — the muscles — 

 are placed at the peripheral extremity of the nerves ; in the case of the centripetal 

 nerves, the organs of reaction — the brain and spinal cord — are found at the 

 central extremity of the nerve-fibres. This theory of the unity of nervous 

 conductibiUty has, moreover, been proved to be correct by the researches of 

 Philipeaux and Vulpian, who have utihzed the experiment of Gluge and Thier- 

 nesse on the union of the central end of the lingual (centripetal), with the 

 peripheral end of the hypoglossal nerve (centrifugal) ; and recently by Bert, in 

 grafting the tail of the Rat into that animal's back. 



