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.

904. Describe the vitreous humor. How does this humor differ from the aqueous? What membrane encloses the vitreous humor?

_Observation._ The structure of this organ can be seen by first freezing the eye of a sheep or an ox; it then can be cut in various directions, and each part separately examined.

905. The MUSCLES of the eye are six in number. They are attached, at one extremity, to the bones of the orbit behind the eye; at the other extremity, they are inserted by broad, thin tendons, near the junction of the cornea with the sclerotic coat. The white, pearly appearance of the eye is caused by these tendons.

[Ill.u.s.tration: Fig. 140. A view of the eye and its muscles. _a_, _b_, _c_, _d_, _e_, Five of these muscles. _f_, The optic nerve. G, The trochlea, or pulley over which one of the muscles pa.s.ses. The bone is seen above and below the eye.]

_Observation._ If the external muscle is too short, the eye is turned out, producing the "wall eye." If the internal muscle is contracted, the eye is turned inward toward the nose. It is then called a "cross eye."

905. How many muscles has the eye? Give their attachments. What causes the pearly appearance of the eye? What does fig. 140 represent? What is the effect if the external muscle is contracted? The internal muscle?

906. The PROTECTING ORGANS are the _Orbits_, _Eyebrows_, _Eyelids_, and _Lachry-mal Apparatus_.

907. The ORBITS are deep, bony sockets, in which the globes of the eyes are situated. They have the form of a cone, the base of which is open and directed forward. The bottom of the orbits is pierced by a large hole which gives pa.s.sage to the optic nerve. These cavities are lined with a thick cushion of fat, in order that the eyes may move in all directions, with perfect freedom and without friction.

908. The EYEBROWS are two projecting arches of integument, covered with short, thick hairs, which form the upper boundary of the orbits.

The eyebrows are so arranged that they prevent the moisture that acc.u.mulates on the forehead, in free perspiration, from flowing into the eye, and also shade these organs from too vivid light.

909. The EYELIDS are two movable curtains placed in front of the eye.

They have a delicate skin on the outside, muscular fibres beneath, and a narrow cartilage on their edges, which tends to preserve the shape of the lid. Internally, they are lined by a smooth membrane, which is reflected over the front of the eye upon the sclerotica. This membrane is called the _con-junc-tiva_. It secretes the fluid that moistens and lubricates the eye, and which causes the eyelids to open and shut without friction.

_Observation._ When the portion of this membrane that is reflected over the globe of the eye, is inflamed, there is frequently a deposition of whitish material, called lymph. This accounts for the films, opacities, and white spots seen upon the eye after the inflammation has subsided.

906. Name the protecting organs of the eye. 907. Describe the orbits.

How are the movements of the eye facilitated? 908. Describe the eyebrows. What does this arrangement prevent? 909. Describe the eyelids. What is the use of the conjunctiva? How are the white spots frequently seen upon the eye accounted for?

910. There are found several small glands on the internal surface of the cartilage, which have the appearance of parallel strings of pearls. They open by minute apertures upon the edges of the lids. The secretion from these glands prevents the edges of the eyelids from being united during sleep.

911. The edges of the eyelids are furnished with a triple row of long, thick hairs, called _eyelashes_, which curve upward from the upper lid, and downward from the lower, so that they may not interlace with each other in the closure of the eyelids. These appendages of the eye, by closing, not only protect it from moisture, but from dust, particularly during sleep. They likewise, by their movements in opening and shutting, spread the lubricating fluid equally over the eye.

912. The LACHRYMAL APPARATUS, which secretes the tears, consists of the _Lachrymal Gland_ with its ducts, _Lachrymal Ca.n.a.ls_, and the _Nasal Duct_.

913. The LACHRYMAL GLAND is situated at the upper and outer angle of the orbit. It is about three quarters of an inch in length, flattened and oval in shape, and occupies a depression in the orbital plate of the frontal bone. Ten or twelve small ducts pa.s.s from this gland, and open upon the upper eyelid, where they pour upon the conjunctiva the lachrymal fluid, or tears. This secretion is maintained while we are asleep, as well as when we are awake. The eye from this cause is kept constantly moist.

914. The LACHRYMAL Ca.n.a.lS commence at minute openings upon the free borders of each eyelid, near the internal angle of the eye, by two small orifices, called _puncta lach-ry-mali-a_, (tear points.) Each of these points communicate with the sac at the upper part of the nasal duct.