Page:Philosophical Transactions - Volume 013.djvu/180

 is perform'd chiefly in the Brain, and these other parts are but the transennæ to it; but also because in an Amaurôsis or gutta serena these parts are free from any indisposition (the Eye appearing, as naturally, without any fault), tho the sight is then wholly lost; and therefore those Fibres of the Optic Nerve must be principally affected, either by being obstructed, or the roots of 'em comprest (about the thalami Optici, by some tumour, or too much pent in by a contraction and extenuation of the outward coats of the Nerves, or by any Confusion or Contortion of the said Fibres.

2. I shew there that the superior Fibre in each thalamus Opticus had the greatest tension, and the inferior the least; as may appear from the former arising from the top of the thalami optici and having the greatest flexure thus ; and the latter arising from the lower part of the aforesaid thalami and having the least flexure thus, as may be judg'd from a view of those parts in the Brain; so that the Correspondence of the former or latter in site and tension caus'd that correspondence or Union in Vision.

3. It may be further noted that the intermediate or lateral Fibres in the same Eye, tho diametrically opposite to one another, are said to differ in tension (by reason of a more considerable flexure of the external then the internal;) whence two Stars or other bodies seen by the collateral Fibres of the same Eye (whilst t' other may be shut) appear distinct and not as one, because they are viewed by discord Fibres of that same Eye; which likewise is so kept in its Orbit by the investing parts, that it can't well be otherwise.

4. I observ'd that the Optic Nerves arose separately from those two Moleculæ of the brain, and besides have a peculiar advantage in rising from these hillocks in this manner; whereas the other Nerves arise from the basis of the brain in a flatter manner, and closer together, so that the extream difference of their rise is very remarkable, and in-