^76 EiiuiKintiiiis of the Means of Aijujlmeiit in the Eye. 



for tlie a<ljunment of the tye, nnd may probably be fufRcient when the lens is remoyed, but 

 not when the eye is enlire. The autlior therefore proceeded to examine what alteration the 

 figure of the human rye might be fufccptiblc of when air was thrown into its cavity, 

 through the optic nerve, fo as to diftend its coats. From the experiments it was found 

 that the axis of vifion was lengthened a fmall degree in the eye of a boy fix years old, forty- 

 five minutes after death, while the tranfverfii diameter and axis from the optic nerve were 

 shortened. A fimilar though lefs clFecl was obferved in the eye of a man 25 years old^ 

 one hour after death ; but uo aheration took place, oy the like treatment, in the eye of i 

 man 50 years old, 20 minutes after death. The objefl of thefe experiments do.-s not clearly 

 appear, as there is no fufpicion of an adion of this kind in the living fubjccl ; but they ap- 

 pear to ilicw that the cornea is the part motl variable from eJanicity ; and when the prcdlirc 13 

 made laterally, and from without, the elongation mull be ftill greater, the adion of the 

 ftraight mufcles being the mod advantageous that could be imagined for this purpofe. 



This lateral preffure, Mr. H. obfcrves, will r.ot only elongate the eye, and incrcafe the con- 

 vexity of the cornea, but will produce an effcft upon the cryflalllMe lens and ciliary pro- 

 ceflcs, pufhing them forward in proportion as the cornea isltretched. For, as thife procefli;s 

 form a complete feptum between the vitreous and aqueous humours, the cavity of the 

 aqueous humour will be always of the fame fize, and the cornea and lens at the fame dif- 

 taiice from each other ; in the accurate produftion' of which effcfts, he fuppofes it likely 

 that the ciliary proccfles may operate by "mufcular action ; an opinion which other fa(SI:s in 

 the courfe of the ledlure lend to confirm. 



The refult of this enquiry, which was not carried on, as he remarks, in fupport of any 

 particular theory, but with the folc view of difeovering the truth, appears to be, that the ad- 

 juftment of the eye is produced by three dllTerent changes in that organ ; an iacreafe of cur- 

 vature in the cornea, an elongation of the axis of vifion, and a motion of the cryftalline lens. 

 Thefe changes, in a great meafure, depend upon the contra£lion of the four (Iraight mufcles 

 of the eye. Mr. Ramfden, from computations grounded on the principles of optics and ge- 

 neral flate of the facSls, ellimates, that the increafe of curvature of the cornea may be capable 

 of producing one-third of the efFe£V, and that the change of place of the lens, and elongation 

 of the axis of vifion, fulhciently account for the other two-thirds of the quantity of adjuft- 

 nient ncceflary to make up tlie whole. 



After this explanation of the nicxle by which the axis of vifion can be elongated, and the 

 convexity of the cornea increafed in the human eye, for the purpofe of its adjuftment, Mr. 

 Home was defirous of applying thefe obfervations to the eyes of other animals. 



Quadrupeds in general mud have their eyes fitted to fee very near objc(f\s, as many of 

 them collect tlieir food with their mouths, in which aftion the objefts are brought very 

 clofc to the eye. Birds are under the fame circumftances in a ftill greater degree with rc- 

 fpect to their food ; but from their mode of life, they alfo require the power of feeing objefls 

 at a great diftance from the eye. Fiflies, from the nature of the medium in which they 

 live, mull have fome other mode of adjulling the eye than that of a change in the cornea, 

 becaufe that fubflance is ponelTedof the fame refra'tive power as the furrounding fluid. 



Qiiadrupeds have three modes of procuring- tlieir food ; one by their fore-paws only, 

 which they ufe like hands, as in the monkey tribe ; tlie fccoud by their fore-pav.'s and 



mouth. 



