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 the lids are bordered with two or more rows of hairs called the eyelashes, which serve both for ornament and for use.  They help to protect the eyes from dust, and to a certain extent to shade them.  Their loss gives a peculiar, unsightly look to the face.

The upper border of the orbit is provided with a fringe of short, stiff hairs, the eyebrows.  They help to shade the eyes from excessive light, and to protect the eyelids from perspiration, which would otherwise cause serious discomfort.

335.  The Lacrymal Apparatus.  Nature provides a special secretion, the tears, to moisten and protect the eye.  The apparatus producing this secretion consists of the lacrymal or tear gland and lacrymal canals or tear passages (Fig. 136).

Outside of the eyeball, in the loose, fatty tissue of the orbit, in the upper and outer corner is the lacrymal or tear gland.  It is about the size of a small almond and from it lead several little canals which open on the inner surface of the upper lid.  The fluid from the gland flows out by these openings over the eyeball, and is collected at the inner or nasal corner.  Here in each lid is a little reddish elevation, or lacrymal caruncle, in which is an opening, communicating with a small canal in the lid which joins the lacrymal sac, lodged between the orbit and the bridge of the nose (Fig. 137).