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at a distance. In hearing, we already refer the cause so completely to a distant object that there is but the smallest possible remnant of a consciousness of sensation in the ear; the sound does not seem to be in the ear, but in yonder bell. Finally, in sight, the impression is so completely projected outward, and the consciousness of anything taking place in the eye so completely lost, that it is only by careful analyses that we can be convinced of its essential subjectiveness.

The order which we have given above is also the order of increasing specialization and refinement of the senses. But only in the two higher senses-only in those senses in which there is no direct contact, but the impressing force is conveyed by means of vibration through a medium-only in these highest senses do we find that, besides the specialization of the nerve-fibers to respond to peculiar vibrations, also an elaborate instrument is placed in front of the specialized nerve in order to intensify the impression and give it more definiteness. It is wholly by virtue of this supplementary instrument that we are able to hear not only sound but music, or to see not only light but objects. The lowest animals in which an optic nerve is found perceive light, but not objects; because, though the specialized nerve is present, the appropriate instrument is wanting. It is on these two higher senses that fine art is wholly and science is mainly founded. The specialized nerve and the instrument for intensifying and making definite the impression are together called the sense-organ. It is of the most highly specialized of these nerves and the most refined of these instruments, the highest of the sense-organs, the eye, that we are now about to treat.

It may be well to bear in mind and keep distinct what may be called the direct gifts of sight, and what are added by the mind as judgments based upon these gifts. The direct data are only light, its intensity, color, and direction. These are incapable of further analysis, and are therefore simple sensations. Outline form may possibly be added, though this may be analyzed into a combination of directions. But solid form, size, and distance, though they may seem to be immediately perceived, are not direct perceptions, but only very simple judgments based on the data given above. We only state these facts now that they may be borne in mind. We hope to substantiate them hereafter.

PART I.

MONOCULAR VISION.

CHAPTER I.

GENERAL STRUCTURE OF THE HUMAN EYE, AND THE

FORMATION OF IMAGES.

SECTION 1.–GENERAL STRUCTURE OF THE 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 hollow 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. 2,17) 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, cc c), 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

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MUSCLES OF THE EYEBALL.-a, optic nerve; b, supe

rior oblique muscle ; c, pulley; d, inferior oblique. The other four are the recti.

called the straight muscles, and two the oblique muscles. The straight muscles all rise at the bottom 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. 20, on page 54, it is seen that the axis of

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