482 SENSORY NERVES. [BOOK in. 



An afferent impulse passing along an afferent nerve may in 

 certain cases simply produce a change in our consciousness 

 unaccompanied by any visible bodily movements; in other cases 

 it may give rise to reflex movements, or modify existing reflex 

 or automatic actions without causing any change in consciousness ; 

 in still other cases it may bring about both results at the same 

 time. An afferent nerve the stimulation of which gives rise to 

 a sensation, and so leads to a modification of consciousness, may be 

 more closely defined as a ' sensory ' nerve. There is however no 

 distinct proof, having regard to the difficulties just mentioned, 

 that the afferent fibres which in the body are commonly used to 

 cause or affect reflex action differ at all in kind from those whose 

 function it is to modify consciousness. On the contrary, such 

 evidence as we have goes to shew that an appropriate stimulus 

 of the same fibre may give rise to one or other or both events ; and 

 that whether the one or the other, or both, events occur depends 

 on the condition of the central organ, and on the relation of its 

 several parts to the afferent nerve. The stimulation of the same 

 nerve (and there are no positive facts which would preclude us 

 from saying ' of the same fibre ') may under certain circumstances, 

 as for instance when the brain has been removed, simply cause 

 a reflex action and under other circumstances give rise merely to a 

 sensation. Hence an afferent nerve is frequently spoken of as 

 a sensory nerve even under circumstances where there is no 

 evidence of consciousness being actually affected, because by a 

 slight change of circumstances the same stimulation of the same 

 nerve might give rise to a distinct sensation ; the substitution of 

 the specific for the general term being justified by the convenience 

 of the former. 



All the spinal nerves are mixed nerves, composed of afferent 

 and efferent, of motor and sensory fibres. When a spinal nerve 

 is divided, stimulation of the peripheral portion causes muscular 

 contraction, of the central portion, a sensation (or a reflex action). 

 At the junction of the nerve with the spinal cord the sensory fibres 

 are gathered into the posterior and the motor fibres into the 

 anterior root. The proof of this, which was first made known 

 by Charles Bell and Majendie, their discoveries forming the founda- 

 tion of modern nervous physiology, is simply 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 sensibility. During the section of the root, or when the 

 proximal stump, that connected with the spinal cord, is stimulated, 

 no sensory effects are produced. When the distal stump is stimu- 

 lated, 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 



