[o] Supra, [Note (i).]
[p] The most celebrated Anatomists differ greatly about the Reason why we see not double with two Eyes. This Galen, and others after him, generally thought to be from a Coalition or Decussation of the Optick Nerves, behind the Os Sphenoïdes. But whether they decussate, coalesce, or only touch one another, they do not well agree. The Bartholines expressly assert they are united, non per simplicem contactum vel intersectionem in homine, sed totalem substantiæ confusionem, Anat. l. 3. c. 2. And whereas Vesalius, and some others had found some Instances of their being disunited; they say, sed in plerisque ordinarie confunditur interior substantia, ut accuratâ disquisitione deprehendi.
But our Learned Dr. Gibson, (Anat. l. 3. c. 10.) saith, they are united by the closest Conjunction, but not Confusion of their Fibres.
But others think the Reason is not from any Coalescence, Contact, or crossing of the Optick Nerves, but from a Sympathy between them. Thus Monsieur Cartes is of Opinion, that the Fibrillæ constituting the medullary Part of those Nerves, being spread in the Retina of each Eye, have each of them corresponding Parts in the Brain; so that when any of those Fibrillæ are struck by any part of an Image, the corresponding Parts of the Brain are thereby affected, and the Soul thereby informed, &c. but see more hereafter under [Note (oo)], from Cartes himself.
Somewhat like this is the Notion of our judicious Dr. Briggs, who thinks the Optick Nerves of each Eye consist of Homologous Fibres, having their rise in the Thalamus Nervorum Opticorum, and thence continued to both the Retinæ, which are made of them; And farther, that those Fibrillæ have the same Parallelism, Tension, &c. in both Eyes; and consequently when an Image is painted on the same corresponding, sympathizing Parts of each Retina, the same Effects are produced, the same Notice or Information is carried to the Thalamus, and so imparted to the Soul, or judging Faculty. That there is such an Ὁμοιοπάθεια between the Retina, &c. he makes very probable from the ensuing of double Vision upon the Interruption of the Parallelism of the Eyes; as when one Eye is depressed with the Finger, or their Symphony interrupted by Disease, Drunkenness, &c. And lastly, That simple Vision is not made in the former way, viz. by a Decussation or Conjunction of the Optick Nerves, he proves, because those Nerves are but in few Subjects decussated, and in none conjoined otherwise than by a bare Contact, which is particularly manifest in Fishes; and in some Instances it hath been found, that they have been separated without any double Vision ensuing thereupon. Vid. Brig. Ophthalmogr. cap. 11. & 5. and Nov. Vis. Theor. passim.
What the Opinion of our justly eminent Sir Isaac Newton is, may be seen in his Opticks, Qu. 15. Are not the Species of Objects seen with both Eyes, united where the Optick Nerves meet before they come into the Brain, the Fibres on the right side of both Nerves uniting there, &c. For the Optick Nerves of such Animals as look the same way with both Eyes, (as of Men, Dogs, Sheep, Oxen, &c.) meet before they come into the Brain; but the Optick Nerves of such Animals as do not look the same way with both Eyes, (as of Fishes and of the Cameleon) do not meet, if I am rightly informed. Newt. Opt. Q. 15.
[q] Nigra est [Uvea] ut radios (ab Oculi fundo ad anteriorem ejus partem reflexos) obumbret; nè hi (ut ait clar. Cartesius) ad Oculi fundum retorti ibidem confusam visionem efficerent. Alia forsan ratio hujus nigredinis statuatur, quòd radii in visione superflui, qui ab objectis lateralibus proveniunt hoc ritu absorbeantur. Ita enim è loco obscuro interdiu objecta optimè intuemur, quia radii tunc temporis circumfuso lumine non diluuntur. Brigg’s Ophthal. c. 3. §. 5.
[r] Admirandum Dei artificium ex diversorum animalium comparatione indies evadit manifestiùs. Mirantur omnes Trochlearem in oculis Hominum & Quadrupedum, & quidem jure: sed admirationem omnem superat, quòd sine Trochleâ oculum movens in Avibus novum genus Trochleæ longè artificiosiùs Nictitandi Membranæ dederit. Blas. Anat. Animal. p. 2. c. 4. ex Stenon.
[Musculum Trochlearem] per intermedium trochleam traductum, nunquam intueor, quin admirabundus mecum, Ὁ Θεὸς, exclamem ὀυ μόνον ἀεὶ γεωμετρεῖ, ἀλλὰ καὶ ἀεὶ μηχανᾶται. I. C. Sturmii Exercit. Acad. 9. de Vis. Org. & Rat. c. 3. §. 4. p. 446.
[] Observare est quod Quadrupedes, qui oculos in terram pronos, ac pendulos gerunt, Musculum peculiarem habent, quo Oculi globus suspenditur——Hoc Musculo Bos, Equus, Ovis, Lepus, Porcus, &c. præditi sunt: hoc etiam Canis instruitur, sed alio modo conformatum habet. Willis de An. Brut. p. 1. c. 15.