900. How are the ciliary processes formed? What does fig. 138 exhibit? 901. Of how many layers is the retina composed? Describe the external layer. The nervous layer.
902. The AQUEOUS HUMOR is situated in the anterior and posterior chambers of the eye. It is an albuminous fluid, having an alkaline reaction. Its specific gravity is a very little greater than distilled water. The anterior chamber is the space intervening between the cornea, in front, and the iris and pupil, behind. The posterior chamber is the narrow space, less than half a line in depth, bounded by the posterior surface of the iris and pupil, in front, and by the ciliary processes and crystalline lens, behind. The two chambers are lined by a thin layer, the secreting membrane of the aqueous humor.
903. The CRYSTALLINE HUMOR, or lens, is situated immediately behind the pupil, and is surrounded by the ciliary processes. This humor is more convex on the posterior than on the anterior surface, and, in different portions of the surface of each, the convexity varies from their oval character. It is imbedded in the anterior part of the vitreous humor, from which it is separated by a thin membrane, and is invested by a transparent elastic membrane, called the capsule of the lens. The lens consists of concentric layers, disposed like the coats of an onion. The external layer is soft, and each successive one increases in firmness until the central layer forms a hardened nucleus. These layers are best demonstrated by boiling, or by immersion in alcohol, when they separate easily from each other.
Observations. 1st. The lens in the eye of a fish is round, 399 like a globe, and has the same appearance, when boiled, as the lens of the human eye.
The vascular layer. 902. Where is the aqueous humor situated? What part of the eye is called the anterior chamber? The posterior chamber? With what are the chambers lined? 903. Where is the crystalline humor situated? With what is it surrounded? Of what does the lens consist? How are these layers best demonstrated? What is produced when the lens, or its investing membrane, is changed in structure?
2d. When the crystalline lens, or its investing membrane, is changed in structure, so as to prevent the rays of light passing to the retina, the affection is called a cataract.
Fig. 139. A section of the globe of the eye. 1, The sclerotic coat. 2, The cornea (This connects with the sclerotic coat by a bevelled edge.) 3, The choroid coat. 6, 6, The iris. 7, The pupil. 8, The retina. 10, 11, 11, Chambers of the eye that contain the aqueous humor. 12, The crystalline lens. 13, The vitreous humor. 15, The optic nerve. 16, The central artery of the eye.