Page images
PDF

Animals ofdivers Forms, according to their pecu

liar Occafions. In some (particularly in Man) it is round:; that being the moft proper Figure for the Position of our Eyes, and the Ufe we make of them both by ?: and Night. In fome other Animals it is of a longish Form : in fòme Tranfverse (24), 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 Ani mals the Fiffure of the Pupil is erećt (25), and alla capitleofopeningwistand stuttingspel:

-------- .

by an Increafe of Light, and when the Objects are nigh. This Motion of the Popil fome say, is estested by the circular and straight Fibres of the Uvea, and some attribute it to the ligamentum ciliare. But I haveno great doubt but that they bởih concur in

that Astion, and that the sigamentum filiare dogh, at the fame

[ocr errors]
[ocr errors]
[graphic]

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 those Noćturnal Animals (in whom generally this erećt Form of the Pupil is) to catch their Prey with the greater Facility in the dark (26), ::::: upwards and downwards, to climb, &c. Thus much for the Tunicks. . . . . . , , -: - } The next Thing I shall take notice of, will relate to the Humours of the : and that only concerning the Mechanifin of the Crystalline Humour; Hot its incomparable Transparency ; nor its exać: lenticular Form ; mor its curious Araneous

[merged small][ocr errors][ocr errors]
[ocr errors]

-

Membrane (27), that conftringeth and dilateth it, * and fo varieth its Focus, (if any fuch Variation

(27) The Tunica Aranea is taken notice of by Frier Bacon, who calls it Tela Aranea, and faith in bác continetur–glaciale vel frystallinum. Rog, Bacon's Perfpeći. Distinft. 2; c. 3. Théwrinkling of this, and the Cornea (as the Skin is of old Perfons) he thinks is the caufe of the Obscurity of the Sight in fuch Perfons, s Bacon Ib par. 2...cap. 2: But this Tunick fome deny, and others allow of: Dr. A. M. of Trinity College, Dublin, (in his Relat. of Anati 0bf; in the Eyes of Animals ina 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 feen it before 'twas exposed to the Air one Minute, - notwithfianding what Dr Briggs faith to the contrary, &c. , But Dr. Briggs his õpinion is, Humor frystallinus, mist aeri diutiùs expositus, vel lenitèr costius (infiar latiis) cuticulam non acquirit : que verò impropriè, Tunica Aranea dicitur, eum f tantum adventitia, ut in Oculo Bovis recens exełło appareat, Briggs's Ophthalmv c. 3. - . ::: : , - ! ! " :: o The Crystalline Humour being of a double Substance, outwardly like a Gelly, towards the Center as confifient as Suet, upon, occafion whereof its Figure may be varied ; which Variation may be made by the Ligamentum Ciliare, Dr. Grew doth, upon these Accounts, not doubt to ascribe 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. l. I. c. 4. Now it is certain by the Laws of Opticks, that fomewhat of this is absolutely necessary to distinst Vision, inafmuch as the Rays proceeding from nigh Objects do more diverge, , and those from difiant Objests less: which requires either that the Crystalline Humour should . . be made more Convex, or more Flat ; or elfe an Elongation or Shortening of the Eye, or of the Distance between the Crystalline Humour and the Retina. : - - { , : But although Dr. Briggs (fo good a Judge) , denies the Tunica Crystallina, contrary to the Opinion of most former Anatomists, yet there is great reafon to conclude he was in a Miftake, in my opinion, from the Observations of the French Anatomifis of the Gystalline of the Eye of the Gemp or Chamois, who fay, The Mem: brana Arachnoides was very thick, and bard, fo that it was easily f'parated from the Crystallinus, p. 145. , t': ', : - . :- The fame Anatomísts also favour the Surmife of Dr. Grew. This [Contraction of the Fibres of the Ligamentum Ciliare on one * fide, and Dilaration on the other] would make us think that thef *** |- H 4 Fibres

there be, as fome affirm with great probability,) nor laftly, its admirable Approach to, or from the | Retina, by help of the Ciliar Ligament (21), , according as Objećts are far off or near, because thefe things

! . . . . . . . . . . Fibres of the Ligamentum Ciliare are capable of contrastion and roluntary Dilatation, like that of the Fibres of the Muscles ; and that this Astion may augment or diminish the Convexity of the a.y. fallinus, according as the need which the Distance of the objesto may make it to have on the Eye, to fee more clearly and difintly. Anat. Descript. of a Bear, p. 49. - , ; , \ : ". . . . - Since my penning the foregoing Notes, having as critically as I could, diffeĉted many Eyes of Birds, Beasts and Fishes, I manifestly found the Membrana Arachnoides, and will undertake to fhew ir ahy one, with great eafe and certainty. It is indeed fò Tramf: rent as not to be feen diffinét from the Cryfailine." But if the , örnea and Uvea be taken off before, or the Mitreous Humour be

hind it, and the out fide of the cryfhalline be gently flashed, the Arachnoides may be feen to open, and the Crystalline willeafily lea; out, and part from the Ligamentum ciliare; which otherwife would not do. For it is by the Arachnoides braced to the Ligamentum Ciliare. . This Membrane or Tuníck, in the Ox, is fo substanstantial and strong, though thin, that it yields to, or finks under the sharpest Lancer, and requíres (for so thin and weak a Memibrane in appearance) a strong Pressure to píerceir. . . . . . . -(28). As Birds and Fishes are in divers things conformable, fs în fome fort they are in their Eye 3 toenable it to correspond to all the Convergencies and Divergencies of the Rhys, which thè Variations of each of the Mediums may produce, o Far this Ser. vice the Tunica Chorosides, (in Fishes) hatň a Musculous Suststanee at the bottorn of ít, lying-round the Optick Nerve at a fmalt Distance from it i by which means I imagine:they are able to contrast and dilate the Choroeides, and thereby to lengthen and fhöften the Eye. For the helping in which Service, I sinagine it is that the choroeides and Scleratică are in a great meafüre parted, that the chorveides may have the greater liberty of asting upon the Hu. mours within. - - : ; ai - .: : · * But in Birds, I have my felf found, that although the chorosides be parted from the sclerotica, yet thechoroeideshathne Mufcie, butin. Read thereof a curiouspestinated work feated en the Optick Nerve, represented in Fig.2. In which c. a. e. b. d. represents the choroeides and Solerotica: a, b, the part of the optiek nervet that is within the Eye : v.v.v. che Pitreous Humour : s. f. g. b. the Pesten $, h. i, the crystalline. For the Reception of this Festen, the optist ' , , Merye

[ocr errors]

Things are what are usually taken notice of; but that which I fhall observeis, the prodigious : and

[ocr errors][ocr errors][ocr errors]

Nerve comes farther within the Eye, than in other Creatures. The Strutture of this Petten is very like that of the Ligamentum Ciliare ; and in the Eye of a Magpy, and fome others, I could perceive it to be musculous towards the Bottom. This Pesten is fo firmly fixed unto, , or embodied in the Fitreous Humour, that the Zitreous Humour hangsfirmly to it, and is not fa eafily parted from it. "By which meansall the Motions of the Pellen are easily coinmunicated to the Witreous Humour, and îndeed to all contained in the choroeides. And forasmuch as thefrystalline is connested to the Pitreous Humour, therefore aifo the Alterations in the Vitreous Humour affest also the crystalline , and the cryfailine is hereby brought nearer unto, or farther from the Retina, as occafion is.

Besides all which Observables in the Ghoroeides and inner Eye, I have alfo found this farther remarkable in the Sclerotica and outer-part of the Eye of Birds, viz. That the fore-part of the Sclerotica is horny and hard, the middle Part thin and flexible, and

Braces intervene between the fore and hind part, running between

the choroeides and Sclerotica. By which means the Cornea and Backpart of the Eye are brought to the fama Gonformiry, that the rest of the Eye hath. . . . . . ; ; ; ; ; ; ; , , , , , , , sa v

The great End and Defign of this fingular and curious Apparatus in the Eyes, both of Birds and Fishes I take to be, 1. To enable those Creatures to fee at all Distançes, far off, or nigh á which (especially in the Waters) requireth a different Conformation of the Eye. In Birds also, this is of great Ufe, to enable them. to fee their Food at their Bill's End, or to reach the utmost Distances their high Flights enable them to view; as to fee over great Trasts of Sea or Land, whither they have Qccafion to fly s ar to fee their Food or Prey, even small Fishes in the Waters; and Birds, Worms, & r on the Earth, when they fit upon Trees, high Rocks, or are hovering high in the Air. 2. Ta eñable hofë

Animals to adapt their Eye to all the various Refrastions of their

adiuns Even the Air it felf. varies the Refrastions, according. as it is rarer or deafer, morę or less comprefied 3 as is manifest, from the learned and ingenious Mr. Lowthorp's Experiment in Phil. Trauf Nr. 257, and fome other Experiments fince of the beforecommended Mr. Hawksbee, both in natural, rarified and com;: ; in each of which, the Refrastions confiantly varied: n exa: Proportion to the Rarity or Denfity of the Air. Vid.

[ocr errors]
« PreviousContinue »