The Greater Work (Opus Majus), Vol. 2

the vitreous humour is of this kind, which is whiter than blood, and less white than the crystalline. And Avicenna Avicenna, a 10th-11th century Persian physician and philosopher. says that the aqueous humour is an excrement of the crystalline, and therefore it is positioned in opposition to its source of nourishment, which is the vitreous humour, and for this reason the crystalline is in the middle of them. And the vitreous humour fills the entire concavity of the nerve up to the common section The optic chiasm., and it is more dense than the anterior crystalline, yet both are transparent, inasmuch as the species The "species" are the sensory impressions or likenesses of objects which travel through the eye. of things pass through them. And the crystalline humour is called the pupil, and in it resides the visual faculty, as in a subject that is first altered, although not radically, since the common nerve is the radical organ, and there vision is completed, as much as the visual faculty can, as the following sections will demonstrate.

Containing three chapters. The first is about the sphericity and the centres of the vitreous and crystalline humours, and of the cornea, and of the aqueous humour and the uvea.

The centres of curvature of the various ocular structures. It is to be considered next regarding the shape of the eye and its parts, and the finding of the centres of the tunics and humours: for these things are absolutely necessary, without which the mode of vision is not clear. Therefore, the whole eye approaches a spherical form, and the tunics similarly, and the humours, because of the commendable properties of the spherical figure, because it is more removed from impediments than a figure having angles, and it is the simplest of figures and the most capacious of isoperimetric Having equal perimeters. figures, as Alhazen Ibn al-Haytham, 11th-century polymath and "author of perspective.", the author of perspective, says here. But in the preceding parts, these properties and others have been touched upon. The anterior crystalline, however, is a portion of a sphere different from the sphere of which the vitreous is a portion: for they are different bodies, and of different transparencies. And the anterior crystalline is a portion of a larger sphere than the sphere of which the vitreous humour is a portion. Whence they are not complete spherical bodies, but are