The

just described. It is also useful in directing the tears through these canals into the nose; and gives out a sebaceous substance like that of the eyelids, which is a good test of strength or weakness, being pale when the constitution is debilitated, but florid in high health. ancients thought it was the source of the tears. In some of the inferior animals, particularly in birds, this spreads over the whole eye, and performs the office of a second eyelid by veiling the eye.

The ball of the eye is considerably smaller than the socket, to give room for its motions, and is not quite globular, but a little elongated. It is made up of several tunics, somewhat like the coats of an onion, which enclose several humours. Next to the transparent skin, which I have described as lining the eyelid and covering the eyeball, there is a hard, opaque, pearly, or bluish white, insensible coat, which surrounds the ball, except the central circle, where the eye is transparent, and is termed the sclerotica.

The descriptions which are given of this coat in books are often very inaccurate, and even contradictory. Some confound it with the reflected skin of the eyelids, already de

scribed, because this closely adheres to it; and others take no notice of the obvious cause of its white colour, and, consequently, give a very partial account of its texture; others, again, confound it with the cornea, or outer transparent coat of the central circle.

The white or sclerotic coat of the eye is a very strong membrane, having the texture and firmness of tanned leather. It is somewhat extensible and elastic, as appears in dropsy of the eye. The sheath of the nerve of the eye is intimately interwoven with this coat, which caused the ingenious M. Bichât to consider it as actually the sheath blown out into a globe. It is this coat which binds the eyeball firmly round, and preserves its figure; and particularly it affords a strong substance for the insertion of the tendons and muscles that move the eye. It is the termination of these muscles which, being spread over the fore part of this coat, gives it that pearly or enamelled appearance, and which is sometimes described as a separate coat. Almost all the terminations of the muscles, in every part of the body, are of this appearance, or somewhat silvery, and nearly insensible, being destitute of nerves. It follows, that both the coat itself, and the ends of the muscles inserted

into it, are insensible. In consumption, the white of the eye, as well as the teeth, become more distinctly of a pearly hue. In some cases of palsy, it has turned quite black. When any of the parts within the eye swell, this coat, from its firmness, resists their expansion, and is, in this manner, productive of great pain.

The middle or choroid coat lies within the white of the eye, adhering to it by an abundant tissue of cells. This middle coat is thin, rather soft, and so full of blood-vessels that its inner surface is velvet-like, from their terminations; and when the eye of a young subject is injected with red wax, it appears like scarlet cloth. It is, indeed, almost a tissue of blood-vessels, particularly on the surface next to the white or sclerotic coat. It is very sensible. The inner surface gives out the pigmentum nigrum, a darkbrown paint, or varnish, which is, in fact, the colour of the whole coat. This is spread over the outer surface of the innermost coat, or expanded nerve, of the eye, and is an important provision in producing sight, as the following facts from Dr. Monro demonstrate :-In the ox, this paint is green; in the cat and owl, it is white and silvery; in the lion, it is golden yellow; in the dog, it is greyish; in man, it is dark brown or

black during youth or manhood, and in old age it becomes deficient.*

We infer from these facts, that the pigmentum given out by the middle coat of the eye is intended to modify the intensity of light.

* The three principal colours of the human eye were well laid down by Aristotle, viz. blue, passing in its lighter tints to what we call grey; an obscure orange, which he calls the colour of the eye in the goat- -a kind of middle tint between blue and orange, and sometimes remarkably green in men. with very red hair and freckled skin; and, lastly, brown in various shades, forming, in proportion to its depth, what we call hazel, dark, or black eyes. The red eyes of the leucathiopic constitution may constitute a fourth division.

These may all occur in different individuals of the same race, or even of the same family; and again, they are sometimes confined to the distinct tribes of the same country, within the limits of a few degrees. Thus, Linnæus describes, in Sweden, the Gothlander, with light hair, and greyish blue eyes; the Fin, with yellow hair, and brown iris; and the Laplander, with black hair and eyes.

Blue eyes, as well as yellow hair, (cœrulei oculi, rutilæ coma,) have characterised the German race from the earliest times; and the same combination is met with, in scattered instances, in the most remote nations. The iris of the negro is the blackest we are acquainted with; so that close inspection is necessary, in living individuals, to distinguish it from the pupil. It is invariably dark in all the coloured tribes of men, as well as in the dark-complexioned individuals of the white variety.

White and pale colours reflect light, while black and deep colours absorb it; hence, animals which prey in the night have this pigment of a paler colour than man, who sees worse in the dark than any other perfect animal. It is from this circumstance that the eyes of cats are observed to gleam in the dark; for they concentrate all the light which falls upon them, and the white parts reflect it back on the objects near them.

The singular variety of the human race called Albinos, who have their hair and skin perfectly white, as well as ferrets, white rabbits, and pigeons, want the pigmentum nigrum altogether; and hence their eyes appear red. It therefore happens that no animal having this peculiarity can see perfectly in bright sunshine or strong light, from the want of its modifying power.

For the purpose of seeing, in a living animal, the blood-vessels of this middle coat, and the beautiful ones also of the next coat in succession, M. Mery plunged a cat into a tub of water. The eye, by this means, was rendered more transparent, and the circulation of the blood could be distinctly perceived.

The expansion of the nerve of the eye,