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In this part are included only those general phenomena which characterize all vision. Besides these there are some which are distinctive of the use of the two eyes as one instrument. These belong to binocular vision.
GENERAL STRUCTURE OF THE HUMAN EYE, AND THE
FORMATION OF IMAGES.
SECTION 1.-GENERAL STRUCTURE OF TIIE EYE.
General Form and Setting.—The eye is nearly spherical in shape, and about an inch in diameter. The socket in which it is set is not a hollow sphere, but an irregular holow cone or pyramid. Evidently, therefore, the deeper and smaller parts of the hollow must be filled with something else. It is filled with loose connective tissue, containing fat. On this, as on a soft cushion, the eyeball rolls with ease in every direction. The eye proper is really behind the skin or outer integument of the face, for the skin which covers the lids turns over the edge (Fig. 2,11) and passes under the lids, becoming here thin and tender mucous membrane; it is then reflected from the back part of the lid to the anterior surface of the white portion of the ball (Fig. 2, a a), then passes forward again over the ball as far as the clear part, or cornea (Fig. 2, ccc), and then entirely over this, although very closely attached. If carefully
dissected off, it would leave the eyeball behind it. This mucous covering of the anterior portion of the eyeball is called the conjunctiva.
Illustrations. In ordinary inflammations of the eye, it is this mucous membrane which is affected, and not the eye proper. Disease of the eye proper is a far more serious matter.
When motes get into the eye they can not go beyond easy reach, viz., beyond the reflection of the mucous membrane, from the lid to the ball, at the points a a.
The Muscles. — We all know the rapidity and precision with which the eye turns in all directions. This is by means of six slender muscles. Four of these are called the straight muscles and two the oblique muscles. The straight muscles all rise at the bottom
MUSCLES OF THE EYEBALL.-a, optic nerve: 6, supe
rior oblique muscle; c, pulley; d, inferior obliqne. The other four are the recti.
of the conical socket, diverge as they pass forward, and grasp the eyeball above, below, on right and left side, just in front of the middle or equator of the globe (Fig. 3). They are called severally superior, inferior, external, and internal rectus. The first turns the ball upward, the second downward, the third to the right, and the fourth to the left, if we are speaking of the right
eye. This is their action expressed generally; but, by reference to Fig. 22, on page 51, it is seen that the axis of 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 turn it inward toward the nose and rotate it on its visual axis inward; while the inferior rectus not only turns the eye downward, but also turns it inward toward the nose and rotates it on its visual 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 visual 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 outward and upward, and at the same time to rotate it on its visual axis outward, i. e., dextrally—or like the hands of a watch—for the right and sinistrally for the left. It is seen that the oblique muscles, besides rotating the eye on its visual axis in opposite directions, co-operate with one another in turning the eye outward. This is neces
sary to counteract the tendency of the superior and inferior recti to turn the eye inward in raising or lowering the plane of vision.
Illustrations of these Actions. If we desire to look upward, we bring into action the two superior recti; if downward, the two inferior recti. In both these cases, however, the oblique muscles must co-operate. In looking upward, the inferior oblique counteracts both the inward turning and the inward rotation produced by the superior recti; in looking downward, the superior oblique counteracts both the inward turning and the outward rotation of the inferior recti. If we look to the right, we bring into action 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 two 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.
Many lower animals, however, use their eyes independently of one another, and are able to turn them in different directions; and to a limited extent the same may be done by man when necessary to accomplish single vision.
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).
SECTION OF THE EYE.-0, optic nerve; S, sclerotic; Ch, choroid; R, retina; v,
ritreous body; Cm, ciliary muscle ; Cj, conjunctiva ; C, cornea ; I, iris ; L, lens; *, aqueous humor ; **, ciliary body or zonule of Zinn.
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