Mathematical Mirror

CHAPTER II.

On the tunics of the eye composed of the three said nerves.

The eye therefore has three tunics or membranes, and three humors, and one web in the manner of a spider's web the retina. And its first tunic is made from the tunic of the inner nerve, which comes from the pia mater, according to all authors, and is expanded from the extremity of the nerve where it enters the foramen of the bone, and it branches out in the manner of a concave net in the first part of itself, which is therefore called the net or retina, according to Avicenna in book 3 of his medical works, and according to Constantinus, having veins and arteries and subtle nerves. Then its second part is thicker, as Avicenna says, and it is expanded spherically to the front of the eye, having a hole in the middle of its anterior part, so that the images of light and color and other visible things may pass through the middle of the eye to the nerve that comes from the brain. For this hole is opposed directly to the extremity of the nerve from which the retina is expanded, and therefore in this entire tunic Alharen says there are two holes: one anterior and the other posterior, which is the extremity of the concave nerve. And this second part of it is called the uvea grape-like tunic, since it is similar to a grape, because it leaves a hole in its anterior part, just as is left in a grape when its stem is removed, as Avicenna says in book 3 of his medical works. And from the tunic of the nerve which comes from the dura mater, according to everyone, the second tunic of the eye is expanded, which has two parts. For the first part is composed of veins and nerves and arteries, and is called the secundina second tunic, because it is similar to the placenta; and the second part is expanded to the front of the eye, and there its manifest part appears, namely the portion of a certain sphere which circles over the extremity of the uvea, and it is like clear horn, and therefore