are required to counteract this defect.

333. The Movements of the Eyes. In order that our eyes may be
efficient instruments of vision, it is necessary that they have the power
of moving independently of the head. The mechanical arrangement by which
the eyeballs are moved in different directions is quite simple. It is done
by six little muscles, arranged in three pairs, which, with one exception,
originate in the back of the cavity in which the eye rests. Four of these
muscles run a straight course and are called the _recti_. The remaining
two muscles bend in their course and are called _oblique_. The
cooerdination of these tiny muscles is marvellous in its delicacy,
accuracy, and rapidity of action.

When, for any cause, the cooerdination is faulty, "cross eye," technically
called strabismus, is produced. Thus, if the internal rectus is
shortened, the eye turns in; if the external rectus, the eye turns out,
producing what is known as "wall eye." It is thus evident that the beauty
of the internal mechanism of the eye has its fitting complement in the
precision, delicacy, and range of movement conferred upon it by its
muscles.

334. The Eyelids and Eyebrows. The eye is adorned and protected by
the eyelids, eyelashes, and eyebrows.

[Illustration: Fig. 134.--Muscles of the Eyeball.

A, attachment of tendon connected with the three recti muscles;
B, external rectus, divided and turned downward, to expose the internus
rectus;
C, inferior rectus;
D, internal rectus;
E, superior rectus;
F, superior oblique;
H, pulley and reflected portion of the superior oblique;
K, inferior oblique; L, levator palpebri superioris;
M, middle portion of the same muscle (L);
N, optic nerve.
]

The eyelids, two in number, move over the front of the eyeball and
protect it from injury. They consist of folds of skin lined with mucous
membrane, kept in shape by a layer of fibrous material. Near the inner
surface of the lids is a row of twenty or thirty glands, known as the
_Meibomian glands_, which open on the free edges of each lid. When one of
these glands is blocked by its own secretion, the inflammation which
results is called a "sty."

The inner lining membrane of the eyelids is known as the conjunctiva;
it is richly supplied with blood-vessels and nerves. After lining the lids
it is reflected on to the eyeballs. It is this membrane which is
occasionally inflamed from taking cold.

The free edges of th