68 PROFESSOR ALISON ON THE NERVES OF THE EYEBALL. 



tively or voluntarily moved are, if not wholly (as SCARPA and others have main- 

 tained), at least almost entirely devoid of any of those filaments which we now 

 regard as the organs of common sensation ; the straight and oblique muscles hav- 

 ing their nerves from the 3d, 4th, and 6th, to the almost complete exclusion of 

 the ophthalmic branch of the 5th.* 



I think we cannot doubt that the reason of this peculiarity, by which the 

 muscles of the eyeball are distinguished perhaps from every other muscle in the 

 body, viz. the absence of purely sensitive filaments in then* composition is that 

 already assigned by VAN DEEN,| viz. that these muscles are intended to be regu- 

 lated and guided in their contractions, not by sensations excited in their own 

 substance, or in parts directly in contact with them, but by the sensations of the 

 Retina ; and I think farther, that to this peculiarity we are to ascribe, both the 

 positive fact, that the movements of these muscles are naturally consentient in the 

 two eyes, so as to preserve the parallelism of the optic axes ; and likewise the 

 negative fact, that we have hardly any power to insulate an act of the will on one 

 of these muscles, so as to move the one eyeball in a different direction from the 

 otljer ; i. e. the left eye, for example, turns inwards when the right eye turns out- 

 wards, because both are habitually guided by the sensations of the retina, which 

 are similarly affected by these movements of the two eyes ; and we have little 

 power of moving either eye independently of the other, because we have hardly 

 any sensations, consequent on the movement of the one eye and not of the other, 

 whereby to guide the efforts of the will for this purpose.^ And this consideration 

 suggests some important reflections on the office of sensitive nerves and of sensa- 

 tions in regard to all movements of voluntary muscles. 



It appears to me, notwithstanding some difficulties recently raised, that the 

 essential peculiarity of all strictly Animal motion is, that it is motion dependent 



* " Cerium et inconcussum ut," says SCARPA, " quinti nervorum cerebri ramum ophtlialmicum, or- 

 bitaui transgradientem, ne minimum quidem filamentum valde conspicuis cscteroquin nervis oculum mo- 

 ventibus addere." (De Gangliis, &c. Isis, 1832.) 



t Do Differentia et Nexu inter Nervos vitee animalis et vitae organic; p. 162. 



J It has been stated by Sir CHARLES BELL, that he believes the 3d nerve to be sensitive as well as 

 motor, because it has an origin from behind as well as from before the grey matter of the crus cerebri ; 

 and although the examples of the portio dura and the spinal accessory nerves (which appear to be purely 

 motor, although originating in part from the posterior portion of the cord) render that inference doubtful, 

 yet I am bound to admit that, according to the statement of VALENTIN,* there is experimental evidence 

 of sensations being felt on irritation of the 3d nerve. But this author is equally confident, from experi- 

 ment, that there is no sensibility in the 6th nerve ;t and it should be remembered that movements are 

 often performed by the 3d nerve, such as rolling the eyes inwards, and raising the eyelid, which are not 

 prompted by the sensations of the retina;, and for the regulation of which sensations in the moving parts 

 themselves may therefore be required. 



* De Functionibus Nervorum Cerebralium, &c. p. 1C. t Ibid. p. 30. 



