AND ITS MODE OF ACTION. 423 



and secondly, a nervous paralysis, in which the affection is confined to 

 the nerve fibres, the muscles retaining their natural properties, and being 

 still capable of contraction under the influence of direct stimulus. 



Identity of Action in the Sensitive and Motor Nerve Fibres. 



The results which are produced by the physiological action of the 

 nerve fibres differ from each other in the two classes to which they 

 belong. The stimulation or excitement of the sensitive fibres produces 

 a sensation, or a sensitive impression in the nervous centre ; that of the 

 motor fibres causes contraction of the muscles to which they are distrib- 

 uted. Moreover, if a sensitive nerve or nerve fibre be divided, stimulus 

 applied to its central or attached extremity still excites a sensation,, 

 while the application of the same stimulus to its separated or peripheral 

 portion produces no apparent result. On the other hand, if a motor 

 nerve be divided, irritation of its attached extremity, which is still in 

 connection with the nervous centre, is without effect ; but irritation of 

 its peripheral portion causes a muscular contraction as before. In other 

 words, the nervous force, in a sensitive nerve, appears to move always 

 in a centripetal direction, that is from without inward ; in a motor nerve, 

 on the other hand, in a centrifugal direction, or from within outward. 

 In the natural condition of the parts, also, the excitement of a sensitive 

 nerve never produces directly any other effect than sensation ; that of a 

 motor nerve only gives rise to the phenomena of movement. 



These facts easily suggest the idea that the two kinds of nerve fibres 

 may be distinct in their properties and modes of action ; that the sensi- 

 tive fibres may be capable of acting only in a centripetal direction and 

 of exciting the phenomena of sensibility ; and that the motor fibres 

 can only act from within outward and transmit a special kind of nerve 

 force, adapted to excite muscular contraction. 



It is evident, however, that the reasons given above are not sufficient 

 to indicate a difference in the activity of the nerves themselves, but only 

 in the sensible results of its operation. In neither case is there any 

 perceptible effect produced in the nerve, but only in the organ with 

 which it is in connection. When a sensitive nerve is excited, the sensa- 

 tion is perceived in the nervous centre ; when a motor nerve is called 

 into activity, the contraction is performed by the muscular fibres at its 

 periphery. It is possible that the condition of the nerve fibres, when in 

 a state of excitement, may be the same in each instance, and that the dif- 

 ference in the effect produced may be due to the different physiological 

 properties of the organs in which they terminate ; just as the conducting 

 wire of a galvanic battery may be made to ring a bell or move an index, 

 according to the mechanism with which its poles are connected. There 

 are certain facts which can hardly bear any other interpretation than 

 this, and which lead to the conclusion that the physiological action in 

 the nerve fibres themselves is not essentially different in different kinds 

 of nerves. 



