THE PUPIL.

A modern traveller states, that he could never understand the declaration in the Song of Solomon, “Thine eyes are like the fish-pools in Heshbon," (vii. 4,) till he had visited the East. But there, as he observed reservoirs for the purpose of containing fish, which, arrayed in their most brilliant colours, darted hither and thither, beneath the brightest rays of the sun, he felt that the changes of which the human eye is susceptible had here a lively and beautiful illustration. The iris is peculiarly sensible to light. It acts, therefore, as a self-adjusting curtain, moderating the rays of light, and preventing too large a quantity being admitted. The shape of the pupil varies in different quadrupeds, though, for the most part, it is round as it is in man. In the cats, that hunt in the gloom, and therefore require all the light that can be made available, it is a long vertical fissure, which becomes round at night. In larger creatures, such as the leopard, the lion, and the tiger, the pupil assumes a round form.

The surface of the choroid coat is covered by a black mucous pigment, to absorb, instead of reflecting, the superfluous rays of light, and thus to secure distinctness of vision. For the same reason, and in imitation of nature, the insides of the tubes of optical instruments have a coating of black. In some quadrupeds, a small

RED EYES.

space is free from this pigment, so that the surface of the choroid, or, as some rather consider it, a thin distinct membrane over the choroid, is seen to glisten at the bottom of the eye. The eyes of cats glare in the light of a candle; an effect produced by the shining of this space without pigment.

Some rabbits, and other animals, have what are called red eyes. In this case the hair is white, and the creatures are called albinos. Some of the human race have these peculiarities. The redness of the eyes in these cases is to be traced to the absence of the black pigment, or rather to the mucous being uncoloured. Thus the vascular structure of the choroid is seen. It is a curious fact, that there appears to be some connexion between the colouring matter of this mucous coat and that of the hair. Where there is a deficiency in the one, there is a deficiency also in the other, as if the secretion were one and the same. Red eyes are always weak, and incapable of enduring strong light.

The inner membrane of all is the retina. The optic nerve at its entrance into the eye divides itself into numerous small bundles of fibrils, and these spread themselves out, and so unite with each other as to form a net-work, which is the outer layer of the retina. From this substance a very large number of fibrils arise,

HUMOURS OF THE EYE.

all directed towards the centre of the eye. These pass through a delicate layer of tissue, containing a minute network of blood-vessels, and from this every fibril receives a sheath, which covers its extremity. The surface of the retina is entirely composed of these little tubercles, which are closely set together. In the retina of a frog, the diameter of these nervous fibres is stated at about th of an inch; while that of the tubercles is about 34th of an inch. In man, they are stated at from the th to 4th of an inch.

800

40

The internal cavity of the eye is filled with three humours. The anterior portion of the eye contains the one called aqueous; it surrounds the iris, which divides the cavity into two chambers, the pupil being the door of communication. Immediately behind the pupil is the crystalline humour, or lens. In man, the lens is like a convex magnifying glass; its texture is like hardened jelly; and it is enveloped with a membranous sac, quite transparent, but not adhering to its surface. If immersed in boiling water it becomes opaque and indurated, and is found to consist of concentric plates, formed by the combination of delicate fibres. The greatest portion of the eye, and the whole of its posterior, is occupied by the vitreous humour. It is of a gelatinous consistence, quite transparent, and

HUMOURS OF THE EYE.

contained in a very fine membrane. This is called the hyaloid, and may be separated at its anterior part by means of a blowpipe, into two plates, so as to form

d

Internal Cavity of the Eye. cc, Aqueous Humour; d, Crystalline Humour, or Lens; e, Vitreous Humour; g, the Retina, an expansion of the optic nerve, f.

a sort of canal. Internally, this membrane forms a cellular apparatus for containing the vitreous humour, so that though the outer part be cut or ruptured, the fluid does not escape.

The globe of the eye is moved by certain muscles, arising from the bony walls of the orbit, and inserted into the sclerotic coat. One, the levator oculi, for raising the globe; another, the depressor oculi, for

MUSCLES OF THE EYE.

moving it downwards; a third, the adductor oculi, for drawing the orbit inwards, towards the inner angle of the eyelids or the nose; a fourth, the abductor, parallel

d

a

Muscles of the Eye. At the upper part of the globe is the levator oculi, at the lower the depressor, and in the middle the abductor. The superior oblique muscle, a, is extended over the upper part of the globe, and gradually assumes the form of the tendon, b, which passes through the pulley, c, fixed to the inner edge of the orbit, d, then returning backwards and downwards, e, is inserted in the sclerotic coat.

on the opposite side of the orbit to the former muscle, for moving the eyeball outwards; and three others, which demand a particular notice.

One of these is the oblique superior muscle, or trochlearis, so called from its tendon passing through a pulley at the internal angular process of the forehead bone, whence it runs obliquely downwards, to be inserted