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.
Fig. 140.
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 401 muscle is contracted, the eye is turned inward toward the nose. It is then called a “cross eye.”