« PreviousContinue »
Eye are prevented, but also it is able with great Readiness and Exactness to apply it self to every Object.
As to the Tunicks of the Eye, many Things might be taken notice of, the prodigious Fineness of the Arachno'ides, the acute Sense of the Retina, the delicate Transparency of the Cornea (w), and the firm and strong Texture of that and the Sclerotica too j and each of them, in these and every other respect, in the most accurate manner adapted to the Place in which it is, and the Business it is there to perform. But for a Sample, 1 shall only take notice of that part of the Uvea which makes the Pupil. It hath been observed by others, particularly by our Honourable Founder (a-), That as we are forced to use various Apertures to our Optick Glasses, so Nature hath made a far more compleat Provision in the Eyes of Animals, to shut out too much, and to admit sufficient Light, by the Dilatation and Contraction of the Pupil (j). Butitdeserveth our especial Remark, that these Pupils are in divers Animals of divers Forms, according'to their
(w) Quis ■verb op'tfex prater Naturam, qua. nihil poteft tjft -caltidiits, tantam Jolerttam perfequi potuijjet in Senfibx*? qua primum Oc*los membranis tenuijjimis vestivit, cr septic; quat primum perlucidas fecit, at per ens cerni poffa:' firmat amem, ut continerentur. Cic. de Nat. Ue-or, 1. z. c. $7. .."•.' (x), Boyl of Final Causes.
(yj It is easy to be observed,, that the Pupil openeth in -dark Places; as also when we look at far distant Objects, but •contracts by an Increase of Light, and when the Objects are nigh. This Motion of the Pupil someVay, is effected by the circular and strait Fibres of the Uvea, and some attribute it to the Ligamentum Ciliare. Yet 1 have no great doubt bit that they both concur in that Action, and that the Ligamer,tum Ciliare doth, at the fame time the Pupil opens or shuts, dilate or compress the Crystalline, 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 shall refer to Mr. Cewper's A, pat. T. 11.
Hz peculiar peculiar Occasions. In some (particularly in Man) ir is round j that being the most proper Figure for the Position of our Eyes, and the Use we make of them both by Day and Night. In some other Animals it is of a longifh Form; in some Transverse (z), with its Aperture large, which is an admirable Provision for such 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 Fissure of the Pupil is erect (««), and also capable of opening wide, and shutting up close. The latter of which serveth to exclude the brighter Light of the Day, and the former to take in the more faint Rays or the Night, thereby enabling those Nocturnal Animals (in whom generally this erect Form of the Pupil is) to catch their Prey with the greater Facility in the dark (bb)t to fee upwards and downwards, to climb, &c. Thus much for the I'unicks.
(«) In Beve, Capri, Equo, Ove, V quilusdam aliis elliptic* tft (Pupilla,) ut to mftgii in hifee sorsan animalibus, am prono in"ctjfuvtilum in agris quint ant, radios later ales ad mala v incomnoda utrinqut devil an da admit tat. Briggs'j Ophtbal. c. 7. $. 6.
Homini ereefa, aliifque, &c. caput engirt, cr quaquaversus tircumspicere Jelitis, flurima fitnul objeila, turn supra, turn infra, turn e lateri utroque——visa excipiuntur; quapropter Oculi Pupilla rotunda ejse debet.—Attamen bovi, 8cc. caput sere semper pronum—gcrentibus, tantitm qua tor am, v paulo a latere obversantur, intttitu opus efi: quapropter Pupilla oblon
ga est, &c. Willis -de Anim. Brut. p. 1. c. I'j.
(aa) Thus Cats (their Pupils being erect, and the shutting 'oF their Eye-lids transverse thereunto) can so close their Pupil, as to admit of, as it were, one only single Ray of Light; • and by throwing all open, they can take in all the faintest Rays. Which is an incomparable Provision for these Animals, that have occasion to watch and way-lay their Prey both by Day and Night.
(bb) There is besides this large opening of the Pupil, in some
nocturnal Animals, another admirable Provision, enabling
'- ■ - - them
The next Thing 1 shall take notice os, 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 lencicular Form; nor its curious arancous Membrane (cc), that constringeth and dilateth it, and
them to catch their Prey in the Dark; and that is a Radiation of the Eyes: Of which Dr. Willis thus; Hujus ufus est Oculi Pupillam, quafi jubare infito, illuminare, ut res noftu, w in tenebris pofitas confpicere valeat: quart in Fete plurimum illustris eft: at Homini, Avibus, c Pifciius dee ft. This Illumination he speaks of, is from the Tapetum, in the Bottom of the Eye, or the shining of the Retina, round the optick Nerve.
Besides which, he faith, the his hath a Faculty also, in some, of darting out Rays of Light, so as to enable them to fee in the Dark: Of which he tells this Story; Not; quendam certbro calidiori prtditum, qui post uberiorem vitii generefi potum in nolle atrata, five lenebrts profundis, literal distineTc legere potuit. Cujus ratio videtur efje, quod fpiritus animates velut accenfi, adeoque ab hac Iride irradiames, jubare infito Medium illummabant. Willis Ibid.
Such another Thing, Pliny tells us, was reported of Tiberius Cesar: Ferunt Tib. Cs.fi. nee alii genitorum mortalium, fuijse naturam, ut expergesattus noflu paulifper, haud alio modo quant luce ciara, contueretur omnia. Nat. Hist. 1, II. C. 37.
So Dr. Briggt: Virum fane calids. indolis novi in Comitatu Bed~
fordienfi degentem, qui oculis felineif donatus est: adeo ut e
]>istolatti~—mirc admodum in loco obscuro ubi eadem tnihi vix apparuit) perlegit. Hujus verb Oculi (mfi quod Pupdlas infignieres obt'muere) ab aitorum formations neutiquam diftrepabant. Ophthal. c. 5. §. ii.
(«) The Tunica Aranea is taken notice of by Trier Bacon, who calls it, Teta Aranea, and faith, in bac continetur—— glaciate vel Cryftallinum. Rog. Bacon'* Perfipecl. Distinct, i. c. 3. The wrinkling of this, and the Cornea (as the Skin is of old Persons) he thinks is the Cause of the Obscurity ot the Sight in such Persons. Bacon Ib. par. 1. cap. z. But this Tuxick some 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. 1681. annexed to his Anat. Account of the Elephant burnt in Dublin, p. <1.) affirms the Tunica Aranea, and faith, / have often seen it before 'twas exposed to the Air one Minute, notwithstanding what Dr. Briggs fifth to the tontrary, &c. But Dr. Briggt his Opinion is, «*»
H 3 twr
so varieth its Focus, (if any such Variation there be, as some affirm with great Probability,) nor lastly
mor CrystaUinus, nisi aeri diutius expofitus, vel Un'tiir cottus (inftar latlis) cuticulam non acqnirit: qu& verb improprie, Tunica Aranea dicitur, cum ji tan turn advtntitia, ut in Oculo Bovis recent exetlo apparent. Briggs'* Ophthalm. e. 3.
The Crystalline Humour being of a double Substance, outwardly like a Gelly, towards the Center as consistent as hard Suet, upon occasion whereof its Figure may be varied; which Variation may be made by the Ligamentum Ciliare; Dr. Crew doth, upon these Accounts, not doubt to ascribe to the Ligamentum Ciitare, a Power of making the Crystalline more Convex, as well as of moving it to, or from the Retina. Sec Grew's Costnolog. Sacr. 1. 1. c. 4. Now it is certain by the Laws of Opticks, that somewhat of this is absolutely necessary to distinct Vision, inasmuch as the Rays proceeding from nigh Objects do more diverge, and those from distant Objects less: Which, requires either that the Crystalline Humour should be made'more Convex, or more flat; or else an Elongation, or shortning of the Eye, or of the Distance between the Crystalline Humour and the Retina.
But although Dr. Briggs (so good a Judge) denies the Tunica Crystallina, contrary to the Opinion of most former Anatomists; yet there is great Reason to conclude he was in a Mistake, in my Opinion, from the Observations of the French Anatomists, of the Crystalline of the Eye, of the Gems or Chamois, who fay, The Membrana Aracbnoides was very thick, and hard, so that it was easily separated from the Cryftallinus,
The fame Anatomists also favour the Surmise of Dr. Grevj, This [Contraction of the Fibres of the Ligamentum ciliare on one side, and Dilatation on the other] would make us think that these Fibres of the Ligamentum Ciliare, are capable of Contraction, and voluntary Dilatation, like that of the Fibres of the Muscles; and that this Action may augment, or diminish the Convexity of the Crystallinus, according as the Need which the Distance of the Objects may make it to have on the Eye, to set mere clearly and distinctly. Anat. Descrip. of a Bear, p. 49.
Since my penning the foregoing Notes, having as critically as I could, dissected many Eyes of Birds, Beasts and Fifties, I manifestly found the Membrana Aracbnoides, and will undertake to (hew it any one, with great Ease and Certainty. Jt is indeed so transparent, as not to be seen distinct from trip QrystaUjne.- Bus if tfcc Cornea arid Vvt$ be taken of before,
Jy, its admirable Approach to or from the Retina, by help of the Ciliar Ligament (dd), according as
or the "vitreous Humour behind it, and the out-side of the Crystalline be gently cut, the Arachnoides may be seen to open, and the Crystalline will easily leap out, and part from the Ligamentum Ciliare; which otherwise it would not do: For it is by the Arachnoides braced to the Ligamentum Ciliare. This Membrane or Tunick, in the Ox, is so substantial and strong, though thin, that it yields to, or sinks under the sharpest Lancet, and requires (for so thin and weak a Membrane in appearance) a strong Pressure to pierce it.
(dd) As Birds and Fishes are in divers Things conformable, so in some sort they are in their Eye; to enable it to correspond to all the Convergences, and Divergences of the Rays, which the Variations of each of the Mediums may produce. For this Service the Tunica Cboroeides, (in Fishes) hath a musculous Substance at the Bottom of it, lying round the optick Nerve, at a small Distance from it; by which Means I imagine they are able to contract, and dilate the Cboroeides, and thereby to lengthen and shorten the Eye: For the helping in which Service, I imagine it is that the Cboroeides, and Sclerotica, are in a great Measure parted, that the Cboroeides may have the greater Liberty of acting upon the Humours within.
But in Birds, I have my self found, that although the Cboroeides be parted from the Sclcrotica; yet the Cboroeides hath no Muscle, but instead thereof, a curious pectinated Work, seated on the optick Nerve, represented in Fig. 2. In which t. a. *. b. d. represents the Cboroeides and Sclcrotica: a. b. the Part of the optick Nerve, that is within the Eye: v. v. v. the ■vitreous Humour: a. f. g. b. the Peclen: h. i the Crystalline. For the Reception of this Ptflen, the optick Nerve comes farther within the Eye, than in other Creatures. The Structure of this Peclen, is very like that of the Ligamentum Ciliare; and in the Eye of a Magpy, and some others, I could perceive it-to be mufculous towards the Bottom. This Peclen h so firmly fixed unto, or embodied in the vitreous Humour, that the vitreous Humour hangs firmly to it, and is not so easily parted from it. By which Means all the Motions of the Peclen are easily communicated to the vitreous Humour, and indeed to all contained in the Cboroeides. And forasmuch as 'the Crystalline is connected to the vitreous Humour, therefore also the Alterations in the vitreous Humour affect also the Crystalline; and the Crystalline is hereby brought nearer uuto, or farther from the Retina, as occasion is.
H 4 Beside^