both sides of a parrot _in_ a cage. The structure of the eyes in Man and the higher animals is very nearly alike, so that the same description will serve for both, but those of insects and the lower orders of animated beings are very different. [Illustration: FIG. 85.--SECTION OF HUMAN EYE. A. Sclerotic. B. Choroid. C. Retina. D. Vitreous humour. E. Aqueous humour. F. Cornea. G. Crystalline lens. H. Pupil. I. Optic nerve. ] The eye consists of various humours of different densities enclosed in membranes (fig. 85); the human eye is spherical for the posterior four-fifths of its circumference, but the front part projects a little, being formed of part of a smaller sphere. The globe of the eye, all but this front part, is enclosed in a firm fibrous coat called the "sclerotic," within which is a membrane full of blood-vessels called the "choroid," this on its inner side is covered with a black substance called the "black pigment," and within this (lining the ball) is the nervous coat or retina, it is the optic nerve so expanded as to form a lining to the membranes of the ball of the eye for the greater part of its extent. The ball of the eye is filled with a transparent fluid, somewhat glutinous, called the "vitreous humour;" stretched across the front part of the ball of the eye is a membrane having a round hole in its centre, called the "iris," coated (like the choroid) with black pigment on its back surface; this membrane (the iris) is the coloured part of the eye, and its transparency or opacity determines the colour of the eye. The pupil is the hole in its centre, it looks black because the black pigment which covers the inside of the ball of the eye (for the retina is transparent) is seen through it; behind the iris and in front of the vitreous humour, is the crystalline lens, it is just the shape of a very thick lens, but is more convex behind than in front, it is quite transparent, and of a density almost approaching to solidity. The front fifth of the ball of the eye is covered in by a dense transparent substance called the cornea, of the form of a watch-glass and quite transparent, between this and the crystalline lens there is a small space called the anterior chamber, filled with a clear watery fluid called the aqueous humour. The eye is moved by six muscles, four straight and two oblique, these turn it in every direction with the greatest accuracy; the front of the eye is defended by the eyelids, which by winking and being moistened by the tears, keep the eye always moist and free from dust.