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 Or´bits, Eyebrows, Eyelids, and Lach´ry-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 passage 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 accumulates 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-ti´va. 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.