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.]
Fig. 19. THE ANATOMY OF THE EYE
From G. Reisch, Margarita philosophiae, Leipzig,? 1503. Showing the seven tunics and three humours of the eye according to the doctrines of Renaissance anatomists.[196]
A great deal of attention was paid by the Arabians to the diseases and the structure of the eye, and the essentials of Manfredi’s description are to be found in Rhazes, Hunain ben Ishak, and Haly Abbas. The tradition presented by these writers passed early into Western science, and is reproduced, for example, in the works of Constantine Africanus and in the well-known anatomy to which the name of Richardus Anglicus (Richard of Wendover) has become attached[197] (cp. Fig. 19). Avicenna’s description of the eye is somewhat different, and gave rise to the tradition reproduced in the works of John of Peckham and of Roger Bacon (Plate [XXXVIII a]), and it influenced the views of Leonardo and even perhaps of Vesalius (Fig. 20). The views on the anatomy of the eye expressed by Rhazes, Hunain ben Ishak, and Haly Abbas were, on the whole, more widely accepted than those of Avicenna.
Fig. 20. THE ANATOMY OF THE EYE