ON THE MECHANISM OF THE EYE. 



m 



their contraction would necessarily lessen 

 the circumference or superficies of the mass 

 that they contain, and round off all its pro- 

 minences, their attachment about the nerve 

 and the anterior part of the eye must there- 

 fore be brought nearer together. (Plate 11. 

 Fig. 85, 86.) Dr. Olbers compares the mus- 

 cles and the eye to a cone, of which the 

 sides are protruded, and would by contrac- 

 tion be brought into a straight line. But this 

 would require a force to preserve the cornea 

 as a fixed point, at a given distance from the 

 origin of the muscles ; a force which cer- 

 tainl}' does not exist. In the natural situa- 

 tion of the visual axis, the orbit being coni- 

 cal, the eye might be somewhat lengthened, 

 although irregularly, by being forced further 

 into it ; but, when turned towards cither 

 side, the same action would rather shorten its 

 axis : nor is there any thing about the human 

 eye that could supply its place. In quadru- 

 peds, the oblique muscles are wider than in 

 man ; and in many situations might assist in 

 the effect. Indeed a portion of the orbicu- 

 lar muscle of the globe is attached so near to 

 the nerve, that it might also cooperate in the 

 action : and I have no reason to doubt the 

 accuracy of Dr. Olbers, who states, that lie 

 effected a considerable elongation, b}' tying 

 threads to the muscles, in the eyes of hogs 

 and of calves ; yet he does not say in what 

 position the axis was fixed ; and the flacci- 

 dity of the eye after death might render such 

 a change very easy, as would be impossible 

 in a living eye. Dr. Olbers also mentions an 

 observation of Professor Wrisberg, on the eye 

 of a man whom he believed to be destitute of 

 the power of accommodation in his life time, 

 and whom he found, after death, to have 

 wanted one or more of the muscles : but this 



want of accommodation was not at all ac- 

 curately ascertiii ned . I measured, in the hu- 

 man eye, the distance of the attachment of 

 the inferior oblique muscle from the insertion 

 of the nerve: it was one fifth of an inch ; 

 and from the centre of vision, not a tenth of 

 an inch ; so that, although the oblique mus- 

 cles do, in some positions, nearly form a part 

 of a great circle round the eye, their action 

 would be more fitted to flatten than to elon- 

 gate it. We have therefore reason to agree 

 with VVinslow, in attributing to them the of- 

 fice of helping to support the eye on that side 

 where the bones are most deficient: they 

 seem also well calculated to prevent its being 

 drawn too much backwards by the action of 

 the straight muscles. And, even if there 

 were no difficulty in supposing the muscles 

 to elongate the eye in every position, yet at 

 least some small difference would be expectjed 

 in the extent of the change, when the eye is 

 ill different situations, at an interval of more 

 than a right angle from each other ; but the 

 optometer shows that there is none. 



Dr. Ilosack alleges that he was able, by 

 making a pressure on the eye, to accommo- 

 date it to a nearer object * : it does not ap- 

 pear that he made use of very accurate means 

 for ascertaining the fact; but, if such an el- 

 fect took place, the cause must have been an , 

 inflection of the cornea. 



It is unnecessary to dwell on the opinion 

 which supposes a joint operation, of changes 

 in the curvature of the cornea, and in the 

 length of the axis. This opinion had derived 

 very great respectability, from the most in- 

 genious and elegant manner in which Dr. 

 Olbers had treated it, and fro t;n being the 

 last result of the investigations of Mr. 



* Pliil, Trans. 1794. in. 



