The second tunic in its front part is called _cornea_ because it resembles horn in its substance and colour; and this covering is transparent, so that the images of visible things may penetrate through it. And it is also solid and large and composed of four membranes, so that being near external things it should not receive hurt. With this [corneal tunic] is united posteriorly another tunic [the third] called _sclerotic_, i.e. hard. These two coverings have their origin in the membrane about the brain, that is in the dura mater, just as the first tunic arises from the membrane over the skull, called _almochatim_. The fourth tunic as to its front part is called _uvea_ [because] it is like a seed of a black grape, and in its midst is a hole called the pupil. Nature made this tunic opaque so that the visual spirit should be conserved and not dissipated by the light outside. Moreover nature made the opening in the tunic that the image might penetrate freely; while it is narrow, so that the visual spirit should be concentrated. Thus when the said pupil, or rather hole, dilates more than usual, either naturally or accidentally, the sight becomes imperfect. [The uveal tunic] joins posteriorly the fifth tunic, called _secundina_ because it is made like the after-birth, i.e. the membrane in which the child is enveloped in its mother’s womb, and it arises from the pia mater. The sixth coating in front is called _arachnoid_ because it is formed after the manner of a spider’s web, and posteriorly it joins the seventh coating, called _retina_, because it is made like a net. Between the uvea and the arachnoid anteriorly there is a humour called _albugineus_, like the white of an egg, to moisten the eye and to preserve the convexity of the cornea. In a dead man this humour dries up, and the cornea falls and is flattened, and then the vulgar say that there appears a curtain before the eyes which is an infallible sign of death. Also this humour holds the pupil open; therefore when it dries up the pupil contracts. Between the two last tunics, i.e. the arachnoid and the retina, which have their origin from the optic nerve, there are two humours. These are the _vitreous_ humour, so called from its likeness to liquified glass, and the _crystalline_ humour, from its likeness to a crystal. This is also called the _grandid_, because it is like a hailstone; and it is somewhat hard and round, but flattened anteriorly where it receives the images of visible things, and posteriorly pyramidal shape and pointed. And here is completed the act of seeing. In the posterior part it is surrounded by the vitreous humours by which it is nourished. The crystalline humour is convex anteriorly and the vitreous posteriorly. And the optic nerves come to the eyes and convey the images seen by the eyes to [the seat of] common sensation and to the other internal faculties. [Folio 12 verso, line 7.] * * * * *