Eye are prevented, but also it is able with great Readiness and Exactnefs to apply it felf to every Object.

As to the Tunicks of the Eye, many Things might be taken notice of, the prodigious Fineness of the Arachnoides, the acute Senfe of the Retina, the delicate Transparency of the Cornea (w), and the firm and ftrong Texture of that and the Sclerotica too; and each of them, in thefe and every other respect, in the most accurate manner adapted to the Place in which it is, and the Bufinefs it is there to perform. But for a Sample, 1 fhall only take notice of that part of the Uvea which makes the Pupil. It hath been obferved by others, particularly by our Honourable Founder (x), That as we are forced to ufe various Apertures to our Optick Glaffes, fo Nature hath made a far more compleat Provifion in the Eyes of Animals, to fhut out too much, and to admit fufficient Light, by the Dilatation and Contraction of the Pupil (y). But it deferveth our especial Remark, that thefe Pupils are in divers Animals of divers Forms, according to their

(w) Quis verò opifex prater Naturam, quâ nihil poteft effe callidius, tantam folertiam perfequi potuifjet in Senfibus ? qua primùm Oculos membranis tenuiffimis veftivit, & fepit; quas primum perlucidas fecit, ut per eas cerni poffet: firmas autem, ut continerentur. Cic. de Nat. Deor. 1. 2. c. 57.

(x), Boyl of Final Caufes.

(y) It is eafy to be observed, that the Pupil openeth in dark Places; as alfo when we look at far diftant Objects, but contracts by an Increase of Light, and when the Objects are nigh. This Motion of the Pupil fome day, is effected by the circular and ftrait Fibres of the Uvea, and fome attribute it to the Ligamentum Ciliare. Yet I have no great doubt but that they both concur in that Action, and that the Ligamen tum Ciliare doth, at the same time the Pupil opens or fhuts, dilate or comprefs the Cryftalline, and bring it nigher unto, or carry it farther off the Retina. For the Structure of the Ligamentum Ciliare, and its two Sorts of Fibres, drawn with the Help of a Microscope, I fhall refer to Mr. Cowper's 4nat. T. II.

Hz

peculiar

peculiar Occafions. In fome (particularly in Man) it is round; that being the most proper Figure for the Pofition of our Eyes, and the Ufe we make of them both by Day and Night. In fome other Animals it is of a longifh Form; in fome Tranfverfe (z), with its Aperture large, which is an admirable Provifion for fuch Creatures to fee the better laterally, and thereby avoid Inconveniencies, as well as help them to gather their Food on the Ground, both by Day and Night. In other Animals the Fiffure of the Pupil is erect (aa), and alfo capable of opening wide, and fhutting up clofe. The latter of which ferveth to exclude the brighter Light of the Day, and the former to take in the more faint Rays of the Night, thereby enabling thofe Nocturnal Animals (in whom generally this erect Form of the Pupil is) to catch their Prey with the greater Facility in the dark (bb), to fee upwards and downwards, to climb, &c. Thus much for the Tunicks.

The

(*) In Bove, Capra, Equo, Ove, & quibufdam aliis elliptica eft (Pupilla) ut eo magis in hifce forfan animalibus, qua prono inceffu victum in agris quaritant, radios laterales ad mala & incommoda utrinque devitanda admittat. Briggs's Ophthal. c. 7. §.6.

Homini erecto, aliifque, &c. caput erigere, & quaquaverfus circumfpicere folitis, plurima fimul objecta, tum fuprà, tum infrà, tum è latere utroquevifu excipiuntur; quapropter Oculi Pupilla rotunda effe debet.- -Attamen bovi, &c. caput ferè femper pronum-gerentibus, tantùm que coràm, & paulo à latere obverfantur, intuitu opus eft: quapropter Pupilla-oblonga eft, &c. Willis -de Anim. Brut. p. I. c. 15.

(aa) Thus Cats (their Pupils being erect, and the shutting of their Eye-lids tranfverfe thereunto) can fo close their Pupil, as to admit of, as it were, one only fingle Ray of Light; and by throwing all open, they can take in all the fainteft Rays. Which is an incomparable Provifion for thefe Animals, that have occafion to watch and way-lay their Prey both by Day and Night.

(bb) There is befides this large opening of the Pupil, in fome nocturnal Animals, another admirable Provifion, enabling

them

The next Thing I fhall take notice of, will relate to the Humours of the Eye, and that only concerning the Mechanism of the Crystalline Humour ; not its incomparable Transparency; nor its exact lenticular Form; nor its curious araneous Membrane (cc), that conftringeth and dilateth it, and fo

them to catch their Prey in the Dark; and that is a Radiation of the Eyes: Of which Dr. Willis thus; Hujus ufus eft Oculi Pupillam, quafi jubare infito, illuminare, ut res nottu, & in tenebris pofitas confpicere valeat: quare in Fele plurimùm illuftris eft: at Homini, Avibus, & Pifcibus deeft. This Illumination he fpeaks of, is from the Tapetum, in the Bottom of the Eye, or the fhining of the Retina, round the optick Nerve.

Befides which, he faith, the Iris hath a Faculty alfo, in fome, of darting out Rays of Light, fo as to enable them to fee in the Dark: Of which he tells this Story; Novi quendam cerebro calidiori praditum, qui poft uberiorem vini generofi potum in notte atrata, five tenebris profundis, literas diftincte legere potuit. Cujus ratio videtur effe, quòd spiritus animales velut accenfi, adeoque ab hâc Iride irradiantes, jubare infito Medium illuminabant. Willis Ibid.

Such another Thing, Pliny tells us, was reported of Tiberius Cafar: Ferunt Tib. Caf. nec alii genitorum mortalium, fuisse naturam, ut expergefačtus noctu paulifper, haud alio modo quam luce clara, contueretur omnia. Nat. Hift. 1. 11. c. 37.

So Dr. Briggs: Virum fanè calida indolis novi in Comitatu Bedfordienfi degentem, qui oculis felineis donatus eft: adeò ut epiftolammirè admodùm in loco obfcuro ubi eadem mihi vix apparuit) perlegit. Hujus verò Oculi (nifi quod Pupillas infigniores obsinuere) ab aliorum formatione neutiquam difcrepabant. Ophthal. c. 5- §. 12.

(cc) The Tunica Aranea is taken notice of by Frier Bacon, who calls it, Tela Aranea, and faith, in hac continetur. glaciale vel Cryftallinum. Rog. Bacon's Perspect. Diftinct. 2. . 3. The wrinkling of this, and the Cornea (as the Skin is of old Perfons) he thinks is the Caufe of the Obfcurity of the Sight in fuch Perfons. Bacon Ib. par. 2. cap. 2. But this Tunick fome deny, and others allow of: Dr. A. M. of TrinityCollege, Dublin, (in his Relat. of Anat. Obf. in the Eyes of Animals, in a Letter to Mr. Boyl, Ann. 1682. annexed to his Anat. Account of the Elephant burnt in Dublin, p. 57.) affirms the Tunica Aranea, and faith, I have often seen it before 'twas expofed to the Air one Minute, notwithstanding what Dr. Briggs faith to the contrary, &c. But Dr. Briggs his Opinion is, H«

H 3

mor

fo varieth its Focus, (if any fuch Variation there be, as fome affirm with great Probability,) nor laft ly

mor Crystallinus, nifi aeri diutiùs expofitus, vel lenitèr coctus (inftar lactis) cuticulam non acquirit: que verò impropriè, Tunica Aranea dicitur, cùm fi tantùm adventitia, ut in Oculo Bovis recens execto appareat. Briggs's Ophthalm. c. 3.

The Cryftalline Humour being of a double Substance, outwardly like a Gelly, towards the Center as confiftent as hard Suet, upon occafion whereof its Figure may be varied; which Variation may be made by the Ligamentum Ciliare; Dr. Grew doth, upon thefe Accounts, not doubt to afcribe to the Ligamentum Ciliare, a Power of making the Crystalline more Convex, as well as of moving it to, or from the Retina. See Grew's Cofmolog. Sacr. 1. 1. c. 4. Now it is certain by the Laws of Opticks, that fomewhat of this is absolutely neceffary to diflinet Vifion, inasmuch as the Rays proceeding from nigh Objects do more diverge, and thofe from diftant Objects lefs: Which, requires either that the Crystalline Humour thould be made more Convex, or more flat; or elfe an Elongation, or fhortning of the Eye, or of the Distance between the Crystalline Humour and the Retina.

But although Dr. Briggs (fo good a Judge) denies the Tunita Cryftallina, contrary to the Opinion of most former Anatomifts; yet there is great Reafon to conclude he was in a Miftake, in my Opinion, from the Obfervations of the French Anatomifts, of the Crystalline of the Eye, of the Gemp or Chamois, who fay, The Membrana Arachnoides was very thick, and hard, fo that it was easily separated from the Crystallinus, P. 145.

The fame Anatomifts alfo favour the Surmife of Dr. Grew, This [Contraction of the Fibres of the Ligamentum Ciliare on one fide, and Dilatation on the other] would make us think. that thefe Fibres of the Ligamentum Ciliare, are capable of Contraction, and voluntary Dilatation, like that of the Fibres of the Mufcles; and that this Action may augment, or diminish the Convexity of the Cryftallinus, according as the Need which the Distance of the Objects may make it to have on the Eye, to fee more clearly and diftinctly. Anat. Defcrip. of a Bear, P. 49.

Since my penning the foregoing Notes, having as critically as I could, diffected many Eyes of Birds, Beafts and Fishes, I manifeftly found the Membrana Arachnoïdes, and will undertake to fhew it any one, with great Eafe and Certainty. It is indeed fo tranfparent, as not to be feen diftinct from the Cryftalline. But if the Cornea and Uvee be taken off before,

ly, its admirable Approach to or from the Retina, by help of the Ciliar Ligament (dd), according as Objects

or the vitreous Humour behind it, and the out-fide of the Cryftalline be gently cut, the Arachnoides may be seen to open, and the Crystalline will eafily leap out, and part from the Ligamentum Ciliare; which otherwife it would not do: For it is by the Arachnoïdes braced to the Ligamentum Ciliare. This Membrane or Tunick, in the Ox, is so substantial and strong, though thin, that it yields to, or finks under the sharpest Lancet, and requires (for fo thin and weak a Membrane in appea rance) a strong Preffure to pierce it.

(dd) As Birds and Fishes are in divers Things conformable, fo in fome fort they are in their Eye; to enable it to corres pond to all the Convergences, and Divergences of the Rays, which the Variations of each of the Mediums may produce. For this Service the Tunica Choroeides, (in Fifhes) hath a mufculous Substance at the Bottom of it, lying round the optick Nerve, at a fmall Distance from it; by which Means I imagine they are able to contract, and dilate the Choroeides, and thereby to lengthen and shorten the Eye: For the helping in which Service, I imagine it is that the Cheroeides, and Sclerotica, are in a great Measure parted, that the Choroeides may have the greater Liberty of acting upon the Humours within.

But in Birds, I have my felf found, that although the Choroeides be parted from the Sclerotica; yet the Choroeides hath no Muscle, but inftead thereof, a curious pectinated Work, feated on the optick Nerve, reprefented in Fig. 2. In which c. a. e. b. d. reprefents the Choroeides and Sclerotica: a. b. the Part of the optick Nerve, that is within the Eye: v. v. v. the vitreous Humour: a. f. g. b. the Pecten: h. i the Crystalline. For the Reception of this Pecten, the optick Nerve comes farther within the Eye, than in other Creatures. The Structure of this Pecten, is very like that of the Ligamentum Ciliare; and in the Eye of a Magpy, and fome others, I could perceive it to be mufculous towards the Bottom. This Pecten is fo firmly fixed unto, or embodied in the vitreous Humour, that the vitreous Humour hangs firmly to it, and is not fo eafily parted from it. By which Means all the Motions of the Pecten are easily communicated to the vitreous Humour, and indeed to all contained in the Choroeides. And forafmuch as the Crystalline is connected to the vitreous Humour, therefore alfo the Alterations in the vitreous Humour affect alfo the Cryftalline; and the Crystalline is hereby brought nearer unto, or farther from the Retina, as occafion is.