The Organ of the Sight.

THe first thing that comes to be handled in this Part of Opticks, is the knowledge of those parts which constitute the Eye, viz. six Tunicles, three Humours, two Nerves, called the Optick, or Visorian Nerves, and seven Muscles; the description whereof appears by the first Figure; not to speak of the Fat, or Glandule, of which the Ball of the Eye seems compacted; but because the Nerves are the principal part from which the Eyes seem to proceed, we will begin with them.

I. The Optick Nerves are produc'd from the foremost part of the Brain, whence after they have proceeded a little way, they all joyn toge∣ther, but soon depart again asunder, so as that they form a Figure, resembling in some measure the Letter X, as is shewed in the second Figure. It is a thing worth the observing, that both these Optick Nerves are, as it were, cloathed with a double Tunicle, whereof one is a Particle of the Membrane of the Brain, which is called the Pia-Mater; the other, of that which is named the Dura-Mater; from the Tunicles, and from the Optick Nerves themselves, proceed those various Tunicles of which the Eye it self consists.

II. From the very substance of this Nerve di∣lated, and (as it were) interwoven with several [ 30] Filaments in form of a Net, ariseth the Tunicle, which thereupon is called the Reticular, or Retine Tunicle, as from the Pia-Mater; that which from the heat of the Husk of a Black-Grape is called the Vveal Tunicle. This Tunicle involves the Eye, but in such a manner, that in the fore-part it hath a Hole directly answering to the Cristalline Humor, through which (as through a Window) are taken the Species transmitted by the Objects; and this is called the Pupilla, or Apple [ 40] of the Eye; though Aristotle, and some others, makes the Pupil and the Cristalline Humor all one. From the Dura-Mater springs that, which from its hardness and transparency together is called the Corneous, and overspreads all the Eye; but as it is continued farther, and closeth the latter part of the Eye, it is called the Sclerotick, and Consolidative Tunicle: But over this there also runs another, which likewise overspreads almost the whole Eye; this is called the Adnata. It [ 50] ariseth from the Pericranium, and joyns the whole Body of the Eye with the Cranium it self, within its proper Cavity; wherefore it is also named Conjunction: But where it extends it self almost over all the foremost part of the Eye, as far as the Iris, it is called the Album Oculi, or White of the Eye, and contains several small Veins and Arteries, as the Vvea doth, by which it receives the Alimentitious Blood.

[ 60] Note, That is this Tunicle there is a Hole, through which the Pupil, and the Vvea is dis∣covered in manner of a Circle, which by reason of its divers Colours is called Iris; and persons are said to have Eyes, Black, Gray, &c. accord∣ing to the Colour which predominates in this Iris.

Page  207 III. In the Eye there are three sorts of Humors, the first and principal is the Glacial, or Cristalline; this is encompassed by a Tunicle, which being as thin as a Spiders-Web, is therefore called Arach∣noides; the second, from the similitude of beaten Glass, is called Hyaloides; the third, is that which is called the Aqueous, or Albugineous Humor, from the likeness of Water, or the White of an Egg.

IV. Each Eye hath seven Muscles, whereof [ 10] two move the Eye upwards and downwards; two Right, and Left; two in circumference; and one by which it is sustained, and the Sight fix'd.