PROFESSOR ALISON ON THE NERVES OF THE EYEBALL. 71 



muscles of the body in the respect above stated, illustrates perfectly the import- 

 ance of the sensitive nerves of muscles, whether these are bound up with their 

 motor nerves, as in most parts of the body, or separated from them, as in the 

 face ; and the importance of those muscular sensations, excited by the contraction 

 of muscles, on the efficacy of which, as a means of acquiring knowledge, the late 

 Dr BROWN dwelt with so much earnestness and ability, but perhaps with some- 

 what exaggerated ideas. 



The office of the sensitive nerves of the voluntary muscles in general, and of 

 the retina and the optic nerve in the eye, in regulating the animal motions, is 

 obviously to furnish the sensations by which the mind is guided, in selecting the 

 muscles and portions of muscles, and in determining the degree of contraction 

 which is requisite for the attainment of any object. And of the necessity of such 

 a regulator in the case of the eye, we have an instructive example when one eye 

 is affected with anaurosis, the effect of which is to prevent that insensible eye 

 from following accurately the movements of the sound eye, when turned in differ- 

 ent directions, and thus to cause occasional and temporary distortion. In fixing on 

 the muscles, or portions of muscles, on which it must act, when it feels certain sen- 

 sations, in order to attain certain objects, the mind sometimes merely yields to that 

 mysterious impulse, independent both of experience and of reasoning, to which 

 we give the name of Instinct ; but in the greater number of cases, in our species, it 

 is guided by experience and education. The sensations which result from any 

 particular muscular action are recollected ; and it is the anticipation, or rather I 

 believe we should say the commencing recurrence, of these sensations, which deter- 

 mines the repetition of the action. Thus the faculty of memory is essential to all 

 strictly voluntary, as distinguished from instinctive, movements ; and the experi- 

 ments of FLOURENS and of HERTWIG instruct us, that it is the cerebellum, not the 

 brain proper, Avhich furnishes the physical conditions requisite for this recollection 

 of muscular sensations. 



Although there appears at first some difficulty in understanding how sensa- 

 tions which are only anticipated, or the beginning of which only is felt, can 

 guide the contractions on which their perfect recurrence is to depend, we shall 

 have no difficulty in conceiving this, if we recollect that it must necessarily be 

 precisely in the same manner that a musician is enabled to go over any piece of 

 music from recollection ; the anticipated sensation is throughout that operation 

 the guide to the motion by which its own recurrence is to be secured. 



In the performance of any such complex successions of muscular movements, 

 we must allow that it is difficult to conceive, that there is not only a continual 

 transmission donmtvards, perhaps to different parts of the body, of certain definite 

 nervous actions resulting from efforts of the will, by motor nerves, but likewise 

 at least as many transmissions upwards by the sensitive filaments, of changes 



VOL. XV. PART I. U 



