656 THE NERVOUS SYSTEM IN GENERAL. 



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 nerves, 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 nervous 

 fibres. This theory of the unity of nervous conductibility has, moreover, 

 been proved to be correct by the researches of Philipeaux and Vulpian, who 

 have utilised the experiment of Gluge and Thiernesse on the union of the 

 central end of the lingual (centripetal) with the peripheral end of the 

 hypoglossal nerve (centrifugal), to demonstrate that the fibres of the former 

 then acquire centrifugal conductibility. 



It is easy to demonstrate that this double property of conduction belongs 

 to all the nerve-fibres springing from the cerebro-spinal axis, centripetal 

 conductibility being peculiar to the superior fibres, and centrifugal conducti- 

 bility to the inferior ones. It is also demonstrated that this conduction acts 

 in either one sense or the other, whatever may be the point on the course of 

 the nerves so stimulated ; as the nerve-tubes possess, throughout their whole 

 length, the property of excitability and conductibility. 



The fibres with centrifugal conductibility constitute the motor nerves ; 

 those whose conductibility is centripetal are the sensitive nerves. But sensi- 

 bility does not exist only in the filaments of the superior roots ; it has also 

 been remarked in the lower roots, and they owe it to the filaments which 

 are given off from the roots whose conductibility is centripetal, and which 

 return to the nervous centres by the motor roots. The sensitiveness evinced 

 by these motor roots is named recurrent sensibility. 



The anatomical and physiological characters of the nerves persist as 

 long as they communicate with the centres. If they are divided at any part 

 of their course, the portion attached to the spinal axis still preserves its 

 properties ; but that situated beyond the section the peripheral end, as it 

 is named degenerates, and becomes incapable of conducting the sensitive 

 impressions, or of transmitting the voluntary motor excitations. 



Now as to the spinal cord. 



Does the medullary axis, which has apparently, in great part, the 

 structure of a nerve, possess, like the latter, excitability and conductibility, 

 those two essential properties of the peripheral nervous system ? 



Excitability is entirely absent in the grey substance. On the surface of 

 a section of the cord, the slightest, or even the most intense irritation of this 

 portion produces no reaction. In the white substance, this excitability can 

 only be easily rendered evident on the surface of the upper bundles or 

 fasciculi, where it is exquisite. With regard to the always limited reactions 

 observed when the excitations are made on the deep part of the fasciculi, it 

 is difficult to say if they result from the excitability of the spinal cord, or 

 that of the nerve roots that traverse the white substance. 



Nervous conductibility is certainly one of the attributes of the spinal 

 cord ; the transmission of excitations of the sensitive nerves to the brain, 

 and the voluntary movements that result from stimulation of the motor 

 nerves, demonstrate that the necessary medium between the nerves and 

 brain the spinal cord possesses conductibility. But does this portion of 

 the nervous system possess no other property? Yes; it may act as a 

 nervous centre, and the following experiment irrefutably demonstrates it : 



