Note cc — BOOK IV.

The Tunica Aranea is taken notice of by Frier Bacon, who calls it, Tela Aranea, and saith, in hâc continetur——glaciale vel Crystallinum. Rog. Bacon’s Perspect. Distinct. 2. c. 3. The wrinkling of this, and the Cornea (as the Skin is of old Persons) he thinks is the Cause of the Obscurity of the Sight in such Persons. Bacon Ib. par. 2. cap. 2. But this Tunick some deny, and others allow of: Dr. A. M. of Trinity-College, Dublin, (in his Relat. of Anat. Obs. 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 saith, I have often seen it before ’twas exposed to the Air one Minute, notwithstanding what Dr. Briggs saith to the contrary, &c. But Dr. Briggs his Opinion is, Humor Crystallinus, nisi aeri diutiùs expositus, vel lenitèr coctus (instar lactis) cuticulam non acquirit: quæ verò impropriè, Tunica Aranea dicitur, cùm si tantùm adventitia, ut in Oculo Bovis recens execto appareat. Briggs’s Ophthalm. c. 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. 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 Cosmolog. Sacr. l. 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 Gemp or Chamois, who say, The Membrana Arachnoïdes was very thick, and hard, so that it was easily separated from the Crystallinus, p. 145.

The same Anatomists also favour the Surmise of Dr. Grew, 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 see more 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 Fishes, I manifestly found the Membrana Arachnoïdes, and will undertake to shew it any one, with great Ease and Certainty. It is indeed so transparent, as not to be seen distinct from the Crystalline. But if the Cornea and Uvea be taken off before, or the vitreous Humour behind it, and the out-side of the Crystalline be gently cut, the Arachnoïdes 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 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 sinks under the sharpest Lancet, and requires (for so thin and weak a Membrane in appearance) a strong Pressure to pierce it.