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the eye is not coincident with the axis of the socket, and, therefore, the action of the superior rectus by itself is not only to turn the eye upward, but also to rotate it a little on its axis inward toward the nose; while the inferior rectus not only turns the eye downward, but also rotates it a little on its axis outward.
The oblique muscles are superior and inferior. The superior oblique (Fig. 3, b) rises like the recti at the bottom of the socket, passes forward, contracts to a slender tendon, passes through a loop situated in the forward part of the socket, on the inner (nasal) and upper side (Fig. 3, c); it then turns upon itself backward and outward, passes over the globe obliquely across the equator, and is attached to the sclerotic, or white coat of the eye, on the outside, a little behind the equator. From its last direction it is evident that its function is to turn the eye outward and downward, and at the same time to rotate it on its axis inward, i. e., sinistrally for the right eye and dextrally for the left. The inferior oblique (Fig. 3, d) rises from the anterior, inner, and lower portion of the socket, passes outward and backward beneath the ball, and, crossing the equator obliquely, is attached to the ball on the outside, a little behind the equator. From its direction it is evident that its function is to turn the eye inward and upward, and at the same time to rotate it on its axis outward, i. e., dextrally—or like the hands of a watch for the right and sinistrally for the left.
Ilustrations of these Actions. If we desire to look upward, we bring into action the two superior recti; if downward, the two inferior recti; if to the right, the exterior rectus of the right and the interior rectus of the left eye; if to the left, the external rectus of the left and internal of the right. If we desire to look at
a very near object, as, for example, the root of the nose, then the two interior recti are brought into action. But we can not voluntarily bring into action the troo exterior recti to turn the eyes outward, nor the superior rectus of one eye and the inferior rectus of the other, so as to turn the one eye upward and the other downward. The reason of this is because such motions, so far from subserving any useful purpose, would only confuse us with double images, as will be explained hereafter, and therefore have never been learned.
Malpositions of the eye, such as squinting, are the result of too great contraction of one of the recti muscles, usually the internal. It is often cured by cutting the muscle, and allowing it to attach itself to a new point.
The Eyeball.—We have thus far spoken only of what is external to the ball, viz., the socket, the muscles, etc. We come now to explain the structure of the ball itself. Suppose, then, the ball be removed from the socket, and the muscles and connective tissue be dissected away; let us examine more minutely its form and structure.
The eye thus separated is nearly a perfect globe, except that the front part is more protuberant (Fig. 4).
1. The outer investing coat, except the small protuberant front part, is a strong, thick, fibrous membrane of a porcelain-white color, called the sclerotic. This is partly exposed in the living eye, and is called the “white of the eye." By its strength, toughness, and elasticity it gives form without rigidity. On this account the ball yields to pressure, but quickly regains its form. It also serves as the basis of attachment for the muscles. If we compare the eye to a globular watch, then the sclerotic represents the outer case.
2. The more protuberant part of the ball is covered
with a thick, strong, but very transparent membrane, called the cornea (C, Fig. 4). It corresponds to the crystal of the watch. Its function is to admit the light, and at the same time to refract it, so as to assist in forming the image, as will be explained hereafter.
i SECTION OF THE EYE.-0, optic nerve; S. sclerotic; Ch, choroid; R, retina; v, vitre
our body; Cm, ciliary muscle ; Cj, conjunctiva; C, cornea ; I, iris; L, lens; *, aqueous humor; **, ciliary body or zonule of Zinn.
3. Running across from the circle of junction of the cornea with the sclerotic, and thus cutting off the more protuberant clear part from the main part of the ball, and thus corresponding in position to the face of the watch, there is an opaque, colored plate called the iris, I. It is the colored part of the eye, black, brown, blue, or gray, in different individuals. This transverse plate is not perfectly flat, but protrudes a little in the middle. In its center is a round hole, called the pupil,
corresponding in position with the hole in the watch face for attachment of the hands. The pupil seems to be jet black, because the observer looks through the pupil into the dark interior of the ball. The function of the pupil is to admit, and at the same time regulate the amount of, light.
4. Linings.—Thus much is visible to the naked eye without dissection. But, if the ball be now carefully opened, the part behind the iris is found to be lined with two thin membranes. (a.) Immediately in contact with the sclerotic is the choroid, a thin membrane, the cells of which are colored with black pigment, which gives it a deep-brown, velvety appearance. Its function is to quench the light as soon as it has done its work of impressing the retina. The anterior portion of the choroid, separated from the sclerotic, drawn together as a curtain, and thickened by muscular tissue, forms the iris already described. Just before separating from the sclerotic to form the iris, it splits into two layers : one, the anterior, goes to form the iris, as already said, while the other, the posterior, is gathered into a circular, plaited curtain, or series of converging folds, which surrounds the outer margin of the lens (to be presently described) like a dark, plaited collar. These plaits, or folds, seventy to seventy-two in number, are called the ciliary processes (Fig. 5, and e, Fig. 19, p. 43). Beneath this dark, plaited collar, and therefore in contact with the sclerotic, is a muscular collar, with radiating fibers, called the ciliary muscle. (6.) Within the choroid, innermost and most important of all, is the retina. This is, in fact, a concave expansion of the optic nerve (0, Fig. 4). This nerve, coming from the brain, enters the eye-socket near its point, penetrates the sclerotic and the choroid, then spreads out within as a thin, concave
membrane of nerve-tissue, covering the whole interior of the ball as far forward as the ciliary collar. Its function is to receive and respond to the impressions of light. Its wonderful structure and functions will be explained hereafter.
5. Contents. The ball thus described is not hollow and empty, but filled with refractive media, as transparent as finest glass. These are:
(a.) Crystalline, or Lens. -Immediately behind the iris, and in contact with it,
g is found the crystalline. It is a flattened ellipsoid, or double convex lens, as clear as finest glass, about one third of an inch in diameter, and one sixth of an inch in SECTION OF EYE.—a, sclerotic; 6, cor
nea; C, conjunctiva; d, iris; e, lens; thickness, firm enough to
s, ciliary muscle behind the dark handle easily, but elastic and ciliary processes; g, retina; h, optic
nerve.' (After Cleland.) easily yielding to pressure. On section it is found to consist of layers, increasing in density from surface to center, as shown in Fig. 5, e, and in Fig. 13, on page 37. The lens is invested with a very thin, transparent membrane, capsule of the lens, which not only invests it, but continues outward as a plaited curtain, to be attached to the sclerotic near the junction of the cornea. The elastic rigidity of the sclerotic pulls gently on this curtain and makes it taut, and the taut membrane in its turn presses gently on the elastic compressible crystalline and slightly flattens it. We shall see the importance of this when we come to speak of the adjustment of the eye for distance.