ttle. At a
certain distance the right-hand bottle will disappear; but nearer or
farther than that, it will be plainly seen.

The vitreous humor fills about four-fifths of the eyeball and prevents it
from falling into a shapeless mass. It also serves to hold the choroid and
the retina in position, and to maintain the proper relations of the inner
structures of the eye.

The iris consists of a framework of connective tissue, the surface of
which is lined by cells containing pigment, which gives color to the eye.

Bundles of involuntary muscular fibers are found in the substance of the
iris. Some are arranged in a ring round the margin of the pupil; others
radiate from it like the spokes of a wheel. When the circular fibers
contract, the pupil is made smaller, but if these fibers relax, the
radiating fibers cause the pupil to dilate more or less widely.

329. The Crystalline Lens. Just behind the pupil and close to the iris is
a semi-solid, double-convex body, called the crystalline lens. It is
shaped like a magnifying glass, convex on each side, but with the
posterior surface more convex than the anterior. In health it is perfectly
clear and transparent, and highly elastic. When the lens becomes opaque,
from change in old age, or from ulcers or wounds, we have the disease
known as _cataract_.

[Illustration: Fig. 129.--Diagram showing the Change in the Lens during
Accommodation.

On the right the lens is arranged for distant vision, the ciliary muscle
is relaxed and the ligament D is tense, so flattening by its compression
the front of the lens C; on the left the muscle A is acting, and this
relaxes the ligament and allows the lens B to become more convex, and so
fitted for the vision of near objects.]

The lens is not placed loosely in the eyeball, but is enclosed in a
transparent and elastic capsule suspended throughout its circumference by
a ligament called the suspensory ligament. This ligament not only
retains the lens in place, but is capable of altering its shape. In
ordinary conditions of the eye, this ligament is kept tense so that the
front part of the lens is flattened somewhat by the pressure on it.

All around the edge, where the cornea, sclerotic, and choroid meet, is a
ring of involuntary muscular fibers, forming the ciliary muscle. When
these fibers contract, they draw forwards the attachment of the suspensory
ligament of the lens, the pressure of which on the lens is consequently
diminished.