744 THE NERVOUS SYSTEM IN GENERAL. 



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

 all the nerve-fibres arising from the cerebro-spinal axis — centripetal c4)ndvMibility 

 being peculiar to the superior fibres, and centrifugal conductibilitt/ to the inferior 

 ones. It is also demonstrated that this conduction acts in either one sense or 

 the other, whatever may be the part 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 are the motor nerves ; those with 

 centripetal conductibility are the sensitive nerves. But sensibiUty does not exist 

 only in the filaments of the superior roots ; it has also been remarked in the 

 lower roots, and these owe it to the filaments which are given off from the roots 

 with centripetal conductibility, and which return to the nervous centres by the 

 motor roots. The sensitiveness evinced by these motor roots is named recurrent 

 sensibility. This sensitiveness has also been demonstrated on the peripheral end 

 of the sensory nerves of the limbs and face (Arloing and Tripier). 



The anatomical and physiological characters of the nerves persist as long as 

 they are in communication 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 stimuli. 



Xow 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 por- 

 tion, 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 

 stimulations 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 

 which traverse the white substance. 



Nervous conductibility is certainly one of the attributes of the spinal cord ; 

 the transmission of stimuli 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 the spinal cord may act as a nerve-centre, and the 

 following experiment irrefutably demonstrates it. 



I will suppose that an animal has had its spinal cord cut across in the lumbar 

 region, and I excite, by pinching, one of the superior roots remaining intact on 

 the caudal portion. The stimulus cannot be conducted to the brain, as this 

 part is isolated from it ; and yet movements take place in the muscles of the 

 posterior limbs. Does it happen that, after section of the medulla, the conductive 

 property of the nerve-fibres which arise superiorly, is interverted and changed 

 into centrifugal conductil)ility ? No ; for after the transverse section of these 

 roots, the irritation of their central end produces exactly the same effects. It 

 must be, therefore, that the stimulation had first reached the medulla, and was 

 then transmitted by it to the muscles by means of the centrifugal-current fibres. 

 And this is really what occurred ; section of the whole of these fibres of the 



