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CHAPTER I.

STRUCTURE OF THE EYE IN MAN AND INFERIOR CREATURES

POINTS

OF DIFFERENCE

AND AGREEMENT DETWEEN THEM.

It would be an interesting task to trace the sense of sight from the little creatures in which it may first be detected, to its most perfect development. At every step of our progress, it would become us to adore the attributes of the great Creator, which are displayed in the humblest as well as the noblest of his works. But for this there would be required space far beyond our present limits, and a minuteness of detail which would not be deemed popular. The attention must, therefore, be directed to a simple description of the structure of the eye, some of the varieties which appear in different creatures, and a few of the most striking facts relating to the function of sight.

THE HUMAN EYE.

Speaking generally, the eye is a globe, or ball, admirably adapted to the purposes intended. Its first, or outer coat, is called the sclerotic. The whole of

[graphic]

The Human Eye. a a, the Sclerotic Coat; 6 b, the Cornea.

the back part of the eye-ball which is sunk within the socket to the edge of the transparent portion, has this covering. It is composed of fibres, interwoven in every direction. In man it is flexible, and endowed with great tenacity. Its use is to protect the important parts within, to preserve the globular figure of the eye, and to afford sufficient support to the muscles by which it is moved. In all vertebrate animals, however, the figure of the eye is not globular; and where this is the case, the sclerotic coat is more or less ossified. In birds, of which the owl is an example, it is surrounded by a bony belt, not unlike the frame of a watchmaker's eye-glass. This is to prevent that tendency to the form of a globe, which fluids on compression naturally

assume,

It appears

The anterior and transparent covering is called the cornea, which

may be compared in shape to a common watch-glass. It is somewhat cup-shaped, and thin at its rim, where it joins the sclerotic coat. to be composed of concentric plates, united by a compact cellular substance. It possesses few blood-vessels and nerves.

The union of the cornea with the sclerotic coat exhibits some diversity in different animals. In man, the cornea slides under the sclerotic coat. In the hare, the sclerotic coat divides at the edges, and like a pair of forceps, grasps the margin of the cornea. In the whale, the fibres of the sclerotic coat pass, in the form of very delicate white lines, into the substance of the cornea. In fishes, the cornea is nearly flat.

Dense as the texture of the cornea may seem, it is porous, and suffers the escape of the fluid behind it, after death. In its healthy state no vessels admitting the red globules of the blood are to be observed in this transparent coat, but vessels, receiving the uncoloured portion, exist in abundance. In inflammation they

SECTION OF THE EYE.

become so enlarged as to admit the coloured globules. Disease, if suffered to continue, always produces opacity, effectually intercepting the rays of light, and thus blindness arises.

Immediately within the sclerotic coat is the choroid membrane ; at its anterior part it gives off a sort of fringe-like structure ; and at the junction of the cornea

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Section of the Eye. a, the Sclerotic Coat; b, the Choroid Membrane,

showing the Black Pigment; c, the Cornea; d, the Iris ; e, the Pupil.

with the sclerotic coat it gives off the iris. The iris is a circular membrane, having a central aperture, termed the pupil. Its colour varies from grey an bluish-grey to hazel, dark hazel, and black. It is the seat of what is commonly called the colour of the eye.

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