904. The VITREOUS HUMOR forms the principal bulk of the globe of the eye. It is an albuminous 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 _hy´a-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. [Illustration: 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 passes. 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 _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.