Part 58 (2/2)

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 cus.h.i.+on 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.

<script>