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from the Horizon with our Eyes. In Brutes also we see their Eyes accommodate themselves in their viewing a single object: But however the rays seem to fall here more readily upon the external Fibres, because of the oblique insertion of the optic Nerve, and therefore they do not so indifferently turn their Eyes to discriminate the Motion of objects as we do, nor is their Sphere of Vision so large.
4. But to proceed to other objections; The case of Cross'd-ey'd persons by birth (that are so from a small contortion of one of the Motory Muscles of the Eye) I have confider'd at the end of the foremention'd [1]Essay, and shewn withal why a Morbid strabismus, or more violent contortion of those Muscles after great convulsions of the Nerves, causes always a double Vision (to which the instances out of Dr. Willis, &c. In p. 176. of those Collections do refer;) which problem was so hard to consider of by Plempius, and others before; and indeed can scarce be solv'd by any other Hyothesis.
5. It has been urg'd That the tension of all the Fibres of the Optic Nerves might be uniform, notwithstanding the greater flexure of the Superior; because these latter might be longer, and consequently might not have a greater stress upon the thalami Optici then the lateral: As, if the arm of a tree grows bent, the Fibres on the protuberant part seem not more stretcht then on the concave side, but to take only a longer compass. To which I answer that sense evinces the contrary in our case (as I shew before), and if any one draws out the Optic Nerve straight forward from the thalamus Opticus, or as it lies in its Natural position, he will plainly see that the top Fibres press more on the subjacent medullary Protuberances then the lateral or, make a deeper
- ↑ In p.177. of the Philos. Collections No. 6.
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