We will now briefly state that the coats of this organ are constituted of a fibrous membrane, T (fig. 95), termed _sclerotic_, which is opaque, except in the anterior portion of the eye, where it forms the transparent _cornea_. The crystalline, C, enshrined behind the cornea, is the convergent lens of the inner chamber; it is covered with a transparent membrane, or _capsule_, and is bathed in two fluids, the _aqueous humour_, between the crystalline humour and the cornea, and the _vitreous body_, a gelatinous humour lodged between the crystalline and the back of the eye. The image of exterior objects which is produced by the passage of light through these refracting surfaces, is received by a nervous membrane, the _retina_, B, formed by an expansion of the optic nerve, N. We must also mention the _choroid_, a membrane lined with a dark pigment, which absorbs the light, and prevents interior reflections, and in front of the crystalline lens, a curtain with an opening, H, called the _iris_, which gives to the eyes their colour of blue, grey, or black. The opening in the centre of the iris is called the _pupil_. [Illustration: Fig. 95.—Structure of the eye.] The penetration of light through the surfaces of the eye is easily demonstrated. An object throws divergent rays on the cornea, a part penetrates into the eye and falls upon the retina, leaving a perfectly retained image of the object. Magendie has proved in the following manner the truth of this mathematical deduction. The eye of a rabbit is very similar to an albino’s; that is to say, the choroid contains no black pigment, but a transparent matter, and when placed before a brilliant object, the image can be seen inverted on the retina. The experiment succeeds also with the eye of a sheep or a cow, if the sclerotic has been lessened. The _optic centre_ of the eye is the point where the secondary axes cross; the _optic axis_ passes through the geometrical axis of the organ, and directs itself spontaneously towards the point that attracts the eye. [Illustration: Fig. 96.—Diagram of mode of vision.]