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Whether the eye has a different nature according to its colors.
Why the black in the eye varies in different people and not in the other animals except in the horse and, rarely, in the dog.
Why the eye has a round shape.
Consequently one asks about the eyes, and first concerning the eyes' anatomy. First, whether the eye has a different nature according to its colors.
1. It seems not. Something that is colorless ought to be receptive to color, and the eye is receptive to color. Therefore, it ought not have its own color.
2. Again, whiteness is caused from an abundance of light. Therefore, where there is more visual light, there will be more whiteness. But the visual power flourishes more in the mean than in the extreme. Therefore, whiteness and blackness, for the same reason, are poorly placed or ordered in the eye.
Second, one asks why the black in the eye varies in different people and not in the other animals except in the horse and, rarely, in the dog. And the contrary to this seems true, because a human is more fixed and stable, as the Philosopher says in that chapter. Therefore his accidents ought to be permanent, etc.
Third, one asks why the eye has a round shape. It seems that it ought to be triangular, "because everything that is seen is seen under an angle," according to the author of the book on Perspective. Therefore, it ought to be triangular, with its base toward the thing seen and the acute angle in the visible eye.
To the first, one must say that the eye is composed of three humors and seven tunics. The principal humor of the eye is called the glacial humor, in which the power of the eye especially flourishes. The second humor is internal and toward the brain. It is the nutriment for the earlier one and is called the vitreous humor, because it resembles liquid glass. The third humor is external, toward the eye's surface, and it is the matter purged from the glacial humor. It is called the albumen, because it resembles the albumen in the egg. Thus the glacial or crystalline or hail-like [ grandineus] humor, which is the same thing, is located between the vitreous humor on the inside and the albumen on the exterior side.
There are three interior tunics and four exterior tunics. The first interior tunic is called the retina, and it wraps about the glacial humor and proceeds from or originates from the optic nerve. The second one is called the secundina and it proceeds from the interior web of the brain, which is called the pia mater. The third is harder than the others, and is called the sclirotica and it proceeds from the dura mater of the brain. This one is inmost toward the brain and is harder than the others in order to prevent the rheums and noxious things from entering from elsewhere. The first exterior tunic is called the "spider's web" [ tela aranea] owing to its subtlety, and it is joined with the interior of the retina and divides the glacial from the albumenlike [ albugineus] humor. The second is called the uvea, which does not cover the entire eye but has an aperture in the middle shaped rather like a grape [ uva] missing the part by which it hangs from the vine. And this exists so that the visible species can approach the glacial humor more freely. The third is called the cornea and it is hard and difficult to cut; thus it is called the cornea [horn-like] owing to its hardness and color. And it is permeable just like horn. And this one is joined to the inner side of the sclerotic, and the uvea is joined with the secundina. The fourth tunic is called the conjunctiva, and it arises from the subcutaneous flesh on top of the cranium, and it is fatty and moist so that it may connect, lubricate, and fatten the other interior tunics. And this is why its circular portion is nearer to white flesh, but because vision requires a consolidation of the visual rays and since white causes fragmentation whereas black consolidates, nature ordained that the part nearer the glacial humor, that is around the pupil, should be black, in order to consolidate the rays. Thus the moisture that is in the middle of the eye has no color apart from the color of water in order to receive all the species of colors as water does. Now the conjoined part is black so that the rays may be consolidated. But the more remote part is white owing to its fattiness, which it receives from flesh as a poultice or lubrication for the tunics. The tela aranea arises from the surface of the optic nerve, the uvea arises from the skin immediately touching the cranium, and the cornea arises from another fleshy tunic of the cranium, existing under the flesh and not very far toward the bottom.
1. On to the arguments. To the first, one must say that the pupil, through which we see, is not a color, although parts joined to it have color.
2. To the second, one must reply that although whiteness may be caused by light it is nevertheless necessary that it exist in a bounded body. But the vision's light exists in the eye in a permeable, transparent, or unbounded body, and this is why there cannot be a middle part that is white, because, if there were, it would scatter the vision.
To the second question one must reply that the black in a human's eye varies in differing ways owing to the different levels of participation in the influence from the brain. Thus the black will be greater or less to exactly the extent that the spirit coming to the black of the eye varies. And because a human's brain varies a great dealsometimes tending to gray, sometimes to black, sometimes to an off-white mottled colorthis is why the black in a human's eye varies. But of all the animals the horse's brain particularly resembles a human brain. Thus the horse, like a human, suffers illness in the head and in other parts from the brain's rheum. And I once had a horse that suffered from rheum, and mucous [coryza] flowed to the nostrils drop by drop. I fumigated the horse often after having spread dried cow dung over coals and after having covered its head, so that, though he had seemed on the verge of death, he escaped. And this is why the black in the horse's eye varies just as it does in the eye of a human. And I had a beautiful little dog at Cologne that had one white eye and one black. And perhaps this was owing to a large quantity of albumen-like humor and the good quality of the spirits and of the visible [visivae] light.
To the argument one must say that a human is more stable in his substantial [qualities] than are other animals. And although the eyes of different people will differ, there is no less stability in a human on this account, because the eye's disposition in the same person is only changed by some accident. So a human is more stable in substantial qualities than in accidental ones, whereas it is the opposite in others.
To the third question one must reply that a spherical shape is especially receptive of, and is better suited for, vision, just as we see in a mirror that those things that are viewed in a flat mirror seem to have their proper size. This is not the case, however, for those things viewed in a convex mirror, because the things seen in a convex mirror do not appear on the surface of the mirror but appear in that spot where the rays that are reflected or refracted from the mirror come together perpendicular [ cathetis] with the eye, just as we said in the third book of the On Meteorology in the chapter on the rainbow.100 Therefore, a spherical shape is best suited for vision because it can hold many things and is less easily impaired and less easily injured, because it touches a plane surface only at a single point.
To the argument one must reply that, although any object is seen in a pyramidal angle, as it were (with the base lying at the thing seen and its apex at the eye), nevertheless the eye itself cannot be angular, because it is not pyramidal but rather of a spherical shape. And the rest are evident.
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