TRANSACTIONS OF THE SECTIONS. 169 



other words, both nerves are at once sensory and motory, although the motory func» 

 tion of the posterior nerves is necessarily slight, because the muscles of the skin are 

 insignificant. 



If we ask what form of sensibility the anterior nerves are likely to excite, the 

 answer cannot be long in forthcoming — it must be muscular sensibility. That we 

 have a Muscular Sense, by means of which we adjust the manifold niceties of con- 

 traction required in our movements, we must all acknowledge ; and it has been shown 

 in a previous communication, that this muscular sense is derived through the mus- 

 cles, and not through the skin ; and further, that it exists after all sensibility to 

 external stimuli has vanished. This muscular sensibility must be derived either 

 through the posterior or the anterior nerves ; but is not derived through the posterior 

 nerves, as Arnold, Brown-Sequard, and the author have proved by the decisive expe- 

 riment of dividing the posterior roots. When these roots are divided, the muscular 

 sensibility is aiFected, but not destroyed; and if any sensibility exist, it must be due 

 to the stimulus of the anterior nerves. Brown-Sequard divided all the sensory roots 

 of the four extremities of a frog, and found that not only did this frog execute its 

 ordinary muscular adjustments, but when its nose was irritated with acid, it rubbed 

 away the acid with its fore-leg. 



The conclusion is that our muscular sensations are derived through the muscle- 

 nerves — there being no other channel for them. The argument against the sensory 

 function of the anterior nerves is this : if we divide the posterior root and irritate 

 the end which is attached to the spinal cord, the animal gives unequivocal signs of 

 sensation ; but if in the same way we divide and irritate the anterior root, the animal 

 gives no sign whatever of sensation. The ends of the nerves which are no longer in 

 connexion with the cord, are irritated, and in the one case no motion is produced, 

 in the other it is. 



Those who demand that an irritation of the anterior root should be followed by 

 the same signs of sensation as follow irritation of the posterior root, demand a kind 

 of evidence which cannot, in the nature of things, be manifested. The sensibility 

 excited by the muscle-nerves cannot be the same as that excited by the skin-nerves, 

 any more than the sensibility excited by the optic nerve can be the same as that by 

 the auditory nerve. No one doubts that the optic nerve is sensory; yet it cannot 

 respond to stimuli of odours, sounds, heat, cold, or touch; whatever stimulates it, 

 will only produce the one special form of sensibility we name Light. Cut it, pinch 

 it, burn it — and no pain is produced, only the sensation of a flash of light. Now, on 

 the supposition that the anterior nerves minister to muscular sensibility, it is obvious 

 that they can only manifest signs of this special sense, and not signs of other senses. 

 There are certain stimuli which awaken muscular sensations ; but whatever awakens 

 them, they will always react in one and the same way. Let us suppose that irrita- 

 tion of the anterior root by pricking, or by galvanism, does awaken this muscular 

 sensibility; by what sign could it betray itself? The irritation produces no pain — 

 no more than irritating the optic or auditory nerve produces pain. It can only 

 produce a sensation, such as precedes or accompanies adjustment of the muscles ; but 

 the muscles which were in direct connexion with this irritated root are now — by the 

 division of the root — removed from its influence, and cannot therefore be adjusted ; 

 and the other muscles are adjusted. What sign, then, could be manifested? Evi- 

 dently none at all. Consequently the experiment, so far from being decisive, does 

 not touch the real question. 



The author then passed to the motory function of the posterior nerves, which, he 

 considered, must necessarily be slight, because function depends on anatomical con- 

 nexion, and unless a nerve be distributed to moving organs, vie cannot expect it to 

 produce motions. Now the posterior nerves are never distributed to the muscles. 

 Schiff has proved that they pass through and along the muscles, and send filaments 

 to the envelopes of muscles, but never terminate in the muscular substance itself. 

 This explains why irritation of a posterior nerve excites no contraction in the muscles 

 if the anterior root be divided. 



But seated in the substance of the skin to which these posterior nerves are distri- 

 buted, there are certain contractile elements— minute muscular fibres — which supply 

 the hair follicles. It is these which are moved by the posterior nerves. Slight as 

 the function of moving such insignificant muscles may be, it is enough to destroy 



