Part 58 (1/2)

897. The CORNEA is the transparent projecting layer, that forms the anterior fifth of the globe of the eye. In form, it is circular, convexo-concave, and resembles a watch-gla.s.s. It is received by its edge, which is sharp and thin, within the bevelled border of the sclerotic, to which it is firmly attached. The cornea is composed of several different layers; its blood-vessels are so small that they exclude the red particles altogether, and admit nothing but serum.

898. The CHOROID COAT is a vascular membrane, of a rich chocolate-brown color upon its external surface, and of a deep black color within. It is connected, externally, with the sclerotic, by an extremely fine cellular tissue, and by the pa.s.sage of nerves and vessels; internally, it is in contact with the retina. The choroid membrane is composed of three layers. It secretes upon its internal surface a dark substance, called _pig-mentum nigrum_, which is of great importance in the function of vision.

899. The IRIS is so called from its variety of color in different persons. It forms a part.i.tion between the anterior and posterior chambers of the eye, and is pierced by a circular opening, which is called the _pupil_. It is composed of two layers. The radiating fibres of the anterior layer converge from the circ.u.mference to the centre. Through the action of these radiating fibres the pupil is dilated. The circular fibres surround the pupil, and by their action produce contraction of its area. The posterior layer is of a deep purple tint, and is called _u-vea_, from its resemblance in color to a ripe grape.

How are this coat and the cornea united? 897. Describe the cornea.

898. What is the color of the external surface of the choroid coat? Of the internal? How is it connected externally? How internally? What does this membrane secrete upon its internal surface? 899. Describe the iris. Of how many layers of fibres is the iris composed? What is the function of the radiating fibres? Of the circular?

900. The CILIARY PROCESSES consist of a number of triangular folds, formed, apparently, by the plaiting of the internal layer of the choroid coat. They are about sixty in number. Their external border is continuous with the internal layer of the choroid coat. The central border is free, and rests against the circ.u.mference of the crystalline lens. These processes are covered by a layer of the pigmentum nigrum.

[Ill.u.s.tration: Fig. 138. A view of the anterior segment of a transverse section of the globe of the eye, seen from within. 1, The divided edge of the three coats--sclerotic, choroid, and retina. 2, The pupil. 3, The iris: the surface presented to view in this section being the uvea. 4, The ciliary processes. 5, The scalloped anterior border of the retina.]

901. The RETINA is composed of three layers: The external; middle, or nervous; and internal, or vascular. The external membrane is extremely thin, and is seen as a flocculent film, when the eye is suspended in water. The nervous membrane is the expansion of the optic nerve, and forms a thin, semi-transparent, bluish-white layer. The vascular membrane consists of the ramifications of a minute artery and its accompanying vein. This vascular layer forms distinct sheaths for the nervous papill, which const.i.tute the inner surface of the retina.

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 alb.u.minous fluid, having an alkaline reaction. Its specific gravity is a very little greater than distilled water. The anterior chamber is the s.p.a.ce intervening between the cornea, in front, and the iris and pupil, behind. The posterior chamber is the narrow s.p.a.ce, 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, 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 pa.s.sing to the retina, the affection is called a _cataract_.

[Ill.u.s.tration: 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.]

904. The VITREOUS HUMOR forms the princ.i.p.al bulk of the globe of the eye. It is an alb.u.minous fluid, resembling the aqueous humor, but is more dense, and differs from the aqueous in this important particular, that it has not the power of re-producing itself. If by accident it is discharged, the eye is irrecoverably lost; while the aqueous humor may be let out, and will be again restored. It is enclosed in a delicate membrane, called the _hya-loid_, which sends processes into the interior of the globe of the eye, forming the cells in which the humor is retained.