The cavity of the globe of the eye in its three posterior quarters is occupied by a substance completely translucent, the vitreous humor. According to most anatomists, it is contained in an envelope called the hyaloid membrane. The vitreous humor and the hyaloid together constitute what is called the vitreous body, which is perfectly adapted to the retina throughout its whole extent, and in front takes the form of the posterior surface of the crystalline. According to those anatomists who admit the existence of the hyaloid, it folds back on a line nearly corresponding to the border of the crystalline, and is continuous* with the ciliary zone of Zinn, or the ciliary processes of the vitreous body; this zone embraces the border of the crystalline, around which it forms the canal of Petit, and adheres intimately to its capsule.
This is the name given to a double convex lens, more curved posteriorly than anteriorly, translucent, and placed vertically in the axis of the eye, so that the axis of the lens corresponds to the centre of the pupil. The crystalline is formed of superposed layers, which are less consistent outside than towards the centre; it is contained in a capsule, which applies itself closely without adhering to it The greater or less convexity of the surfaces of the crystalline modifies the power of the eye, determining whether the vision is long or short, i.e. presbyopic or myopia The opacity of the lens, or of its capsule, forms the disease called cataract. We have already stated that its edge is set into the zone of Zinn, to which its capsule adheres.
Formerly a certain space was supposed to exist between the crystalline and the iris; this was called the posterior chamber of the eye. We now know that the posterior surface of the iris is in direct contact with the anterior surface of the crystalline, and the posterior chamber is only an imaginary space. The interval which divides the iris from the cornea is the anterior chamber, which is filled with a fluid called the aqueous humor, translucent like the vitreous but less dense; it is secreted by the ciliary processes.