5^8 PtctiVtcrtit'us cf Fifion In Fijhs, Hi. 



In tlie fycs of fiflics tlic ciliary proccfles arc entirely wanting. The cryflslline Itns If 

 fpherical and inibcildcd in the vitreous humour, which is inclofed in cells of s Itrocgcr Ic-K- 

 turc th.in in other animals. 



The iiis does not admit of motion; tliis is talccn notice of by H.jllcr; and the rcafon 

 probably is, that the liglit In water is nt"ver too flrong for the eye to bear. 



There is a mufclc fnuatcd between the retina and the fclerotic coat, which is, as Mr.IIomc 

 thinks, common to all fifli. lirn mufclc is particiihirly defcribiid by Mr. IIalltfr;niid its ufc 

 15 fiatcd to be tliat of bihigiiig the retina nearer the cryflallinc lens for the purpofe of fee- 

 ing ohjecls at a greater diflance. iMr. Hunter called it the choroidc mufclc, and h.is pro-' 

 fcrvcd fcvcral preparations of it. 



This mufcle I'.as a tendinous centre rcuiul the optic.ncive, .it which p,irt it is attaclicd to 

 the fclerotic co.it ; the mufcular fibre: are fliort, and go oiT from the central tendon in all 

 dircflions ; the (hape of the mufclc is nearly that of a horfe-flioc ; anteriorly it is attached, 

 to the choroidc coats, and by nic.ms of that to the fclerotic. Itj aiflioii tends evidently to 

 bring the retina forward.<t; and in general the optic nerve in fifhes makes a bend where it ca- 

 ters the eye, to admit of this motion without the nerve being ftrctchcd. 



In thofe fillies ih.n have the fclerotic coat completely covered witli bone, the wliole adjulL- 

 ment lo great diftances mud be produced by tlic aftion of the choroide mufcle ; hut in the 

 others, which ave by far the greatei: number, this efFe£l will be much afTifted by the adion 

 of the ftrait muftles pitlling the eye-ball againft the focket, and comprelTuig the poftcrior 

 part, which, as it is the only membraneous part in many fiOics, would appear to be formed 

 fo for that purpo/e. 



In fidies, the eye in its natural eafy ftate appears to be adjufted to near objects, requiring 

 fomc ch.uige to a-.lapt it to fee diftant ones \ in thio refpetl ditTeving entirely from the bird, 

 tlie quadrujicd, and the human. 



The preceding obfervations, on the Itruflure of the eye, indicate tv.'o methods of adjuftin^ 

 the eye ; one for feting in air, the other for feeing in water ; and tiiat the cryftalline lens, as 

 the mod confpicuous part, appears to have engrofled the whole attention of former enquirers, 

 who in general do not feem to have paid fufficient attention to the elongation of the axis and 

 change of curvature in the cornea. That the axis of vifion is really lengthened, and the lens 

 moved forward, appears highly probable from the whole of the fails ; and fince the elonga- 

 tion of the eye, the change of pofition of the crydalline, or the alteration of curvature in the 

 cornea, have feverally appeared to be the caufe of the requlfite adjuftment, the combination 

 of all thti. ,; muft undoubtedly be fufficient to efie£l the purpofe. 



In the Philofophical Tranfaclions for 1797, page J, Mr. Home explains certain morbid 

 affcilions of the llrait tpufcles and cornea of the eye, and confiders their treatment. He 

 dates the ufes of the mufcles'to be ; firll, that of moving the eyc-balls in diifercnt dire£lions ; 

 fecondly, that of caufing the two eyes to correfpond ; and thirdly, that of producing ad- 

 juftment by their lateral prcflurc. Imperfection in any one of thefe different a£l ions muft 

 be confidered as a difeafe. Three different cafes occur in prackicc : namely, indiftincl vifioO) 

 double vifion, and fquinting. 



Indiftinft vifion is explained to confift of an inability in the mufcles to fupport that de- 

 fte'; of tcnfion which is rcquifite to adjuft the eye to near objects. In this morbid affctliori 

 vl the ftraight mufcles they may be capable of performing all the intermediate contra£lionj 



