But its internal structure is still more admirable. The globe of the eye is almost spherical and measures nearly one inch in diameter. Fig. 1 is a view of the eyeball, showing the details of its structure; the various membranes surrounding it have been cut away in order that it may be better examined. If we commence our examination by the exterior portion of the front, we shall first find immediately beneath the eyelashes a perfectly transparent membrane (C), called the _cornea_. It is a prolongation of the hard opaque external coating of the eye, called the _sclerotic membrane_, and marked S in the figure. The cornea is sufficiently hard in its nature to present a strong resistance to any violence from without. Immediately beneath the cornea and in contact with it is the _aqueous humour_, a thin transparent liquid occupying a small portion of the front of the eye. Next comes the _iris_, a circular disc perforated with a round hole in the middle, and coloured with various shades of blue, brown, and grey. [Illustration: Fig. 1.—Section of the Eye. ] The opening in the centre, which appears like a black spot when the eye is examined, is not really an object, but simply an aperture, capable of changing its size according to the quantity of light striking the eye. This change of size in the opening, or pupil, as it is popularly called, is effected by the contraction or expansion of the iris, which thus possesses the peculiar property of exactly proportioning the amount of light that enters the eye, so that there is never too much or too little. It is through the pupil that the rays of light proceeding from the various objects around us pass into the interior of the eye, and form an image upon the retina, as will be afterwards explained. Immediately behind the pupil is O, a bi-convex lens to transmit the rays of light to the retina. It is generally called the _crystalline lens_. From the crystalline lens to the back of the eyeball, is a space more or less globular in form, containing a gelatinous diaphanous mass somewhat resembling white of egg in appearance, and called the _vitreous humour_. Behind the vitreous humour, and immediately opposite the pupil and lens, is the most delicate and important of all the membranes of the eye, the _retina_, which serves as a screen whereon are received the images of the objects around us. This membrane is an expansion of the optic nerve N leading from the brain, and lines the whole of the interior of the eye. The eye is also enveloped in a second membrane (C), called the _choroid_, which is impregnated with a black pigment. Round this is wrapped a third membrane, the _sclerotic_ (S), which unites with the cornea in front of the eyeball.