Bin 



B I R 



ax 

 ot 



is of the anterior portion being twice as great as that of the mate connexion with it. In ftructiire, however, it is per- 

 her. The obviou. confenucnce of this fi^nre of the globe of feftly funila, to ll.e choroidcs. The form of this part vane. 

 the eye is to a!lo«- room for a greater proportion of the aqne- i" different fpecies ; in gceral, .t con ifts of a membrane 

 ous flmd, and for the removal of the chryllalUi.elei.s fron, the folded backwards and for«-ard3 on iticlf, like the plaits of 

 feat of the fenfat.on, and thus produce a greater cor.vcigence a garment, and prelenting, w'hen viewed on the hde, lome- 

 of the rays of light, by which the animal is enabled to d.f- thing of the appearance ol the teeth of a comb ; on whicli 

 cem the objeds placed near it, and to fee with a weaker account, the name o( p<-chn has been applied to it. In the 

 light 1 and hence o'.vls, «i,ich recj^nc this fort of vifion fo frull.-ious birds, the folds of the peden are larger, and ccU 

 much, poffefs the Itrudure fitted to cfftd it in fo remark- Icded towards the point, givmgit a refemblaace to a purle. 

 able a degree. "^^^ Parifiau academicians therciore, in taking their defcnp- 

 TV<: fcltrotic coat of the eye in birds is thin and flexible on tion of this part from the oJJrich, called it the marfupium wi- 

 the poilerior part, but anteriorly its form is maintained by srum, by which name it is Hill very comnwnly known. 

 the intcrpofition of a number of bony plates between its The plicated membrane proceeds into the fubftance of 

 layers. Thefe vary from thirteen to twenty, and are ar- the vitreous humour, and ufually becomes attached to 

 ranged in a circle immediately behind the cornea, with tiieir the poaerior part of the capfiile of the cryftaliine lens a 

 edges overlappii.g eacli otlier. Thev are commonly flat thin little to one fide, /n fome i-.ilfances it does not come into 

 fc.iles, and nearly of a fquare figure, but become elongated immediate contad with the capfule of the lens, but ends a 



tcrior fphere. The fcale's are capable cf a degree of luc- ven in the great horned oiul. 

 tion upon each other, v.-hich is, liowcvcr, reftraincd within The fuiidions of the plicated membrane have been often 

 certain limits bv the attaclimcnts of their anterior and difcnd' d, but ftlU remain involved in fome degree of doubt. 

 polferior edges to the fclerotic coat; and by their being Halier, and others, confider it as the medium through which 

 bound togrtliir with a tough ligamentous ftibllance, which the veficls are condue'ted to the cryftalli;:e hns ; but tl'.cre 

 feems to be the continu.aion of the fclerotic between tiic appears no reafon fir fuch a piovifion to exift in birds. Pe- 

 cdges that overlap each other. tit was of opinion that it abforbed the lateral rays of light, 

 The cornea pofielTes the fame ilrndure as in mammalia, in order that objeds placed immediately before the eye 

 but dilTers with refped to form. When the poilerior part might be more dillindly feen ; which is highly improbable, 

 of the eye is comprefled by the mufcles, the humours ai--e as the fcope of vilion is full as exteniive in birds as in other 

 urged forwards and dillend the cornea; which, at that animals. The bell fupported tneory on this fubjc6t is Mr. 

 time, becomes much more prominent in moll birds than it Home's. The plicated membrane, according to his experi- 

 is ever obferved in mammalia ; and under fuch circum- ments, poffefles a contradile power, and affords the means- 

 fiances, the eye is in a Hate for perceiving near objeds. of withdrawing the lens from the anterior part of the eye, 

 When the mufcles are quite relaxed, the contents of when the organ is adapted to the perception of remote ob- 

 thc eye-ball retire to the poflcrior part, and the cornea jeds ; thus ading in concert with the change of figure in 

 becomes flat, or even deprcfied : this is the condition in the cornea. Experiments, however, upo.n the operations of 

 which we always find the eye of a dead bird, but we the eye are fo delicate in their nature, that they are feldora 

 can have no opportunity of perceiving it during life. It is to be relied upon, and accordingly in different hands they 

 only pradifed for the purpofc of rendering objeds- vifible have afforded very different refults ; but the dodrine of the 

 that are placed at an extreme dillance. From the mnfeularity of the plicated membrane is almoff proved by 

 well known effeds of form upon refracting media, it mull its feeming neceffity for the explanation of the powers of ad- 

 be prelumed, that the cornea poffelTes very little, if any, aptation of the eyes to different diffances, which birds pof- 

 convexity, when a bird, which is foariiig in the higher re- fefs in a degree far fuperior to all other animals ; and it 

 gions of the air, and invifiblc to us, difcerr.s its prey upon feems fair to infer that if the accommodation of vilion de- 

 the earth, and dcfeends with unerring flight to the fpot, as pends upon the motion of the cornea, and the receflion cf 

 is curtomary with many of the r.ipacinus tribe. the lens in thofe animals which are fo eminently endowed 

 There are other circumftances in the anatomy of the eye with it, fimilar means are employed for the fame purpofc 

 of birds, which have been fuppofed to concur with the ex- in the other claffes which poffefs the faculty in a lefs degree, 

 traordinary variation in the figure of the cornea, in producing For the more ample dilcuffion of this fubjed, fee IMr. 

 its cap icity for the perception of remote objeds : thefe will Home's ledures on mufcnlar motion, publifhed in the Phi- 

 be mentioned in their proper place. lofophical Tranladions for the years 1794, '795> ^'^^ I79<5; 

 The r/.oro;</ CM/ differs in no materi'al point from that of Dr. Olbers' " De oculi mutationibus internis," Gutting, 

 the human fubjed. The r;7;ary yir-^^j^^j of the choroides are 1780 ;" The Effays cf Petit, Mem. de I'Acad." 1735, 



Till ' ' ' ' " ~~ 



very Imall and fliort ; being merely ierrated ilrix. 

 appears to be no iapctum. 



The iris is fometimes of brilliant colours, which are em- 

 ployed by naturalifts as diftinguiftiing fpecilic charaders of 

 birds. Parrots have the power of voluntarily producing a 

 great degree of motion in the iris. It does not appear, 

 however, tlrat other biids are capable of commanding the 

 motions of this part 



Tl 

 oftl 



p. 163. 1736, p. 166; " Obfervations on the Eyes of 

 Birds," by Mr. Pierce Smith, Phil. Tranf. for 1 795 ;" 

 and " Dr. Young's Ledure on the Mechanifm of the Eye," 

 Phil. Tranf. i8oi. 



The optic nerve paffes through an oblique (heath in the 



back of the fclerotic coat, during which it changes from a 



round to a flattened fliape, and, as fuch, enters the eye, pre- 



fenting on the infide an elongated white line, inllead of a 



Vcpcclen, or plicated memhrane, is the mofl Angular part round diflc, from which the retina is produced. The origin 



re ftrudure of the eye of birds. It appears to grow of the plicated membrane covers the entrance of the optic 



from the choroides where the optic nerve penetrates that nerve. 



Qoat ; but on cloftr iiifpedion it is found to have no inti- The humours, as well as the (hape of the eye aOd the flruc 



ture 



