Our description is not yet complete, however. The eyeball, at least the posterior chamber, has an innermost lining, called the retina 00. The retina lines nearly the whole of the inner sur face of the posterior chamber, lying on the choroid coat. It is, consequently, with the retina that the vitreous humour is in contact. The retina is the nervous coat of the eye; it really forms the terminal organ (p. 441) of the sense of sight. It is a very thin, soft, white membrane. If the fresh eye of a sheep or ox be opened, and the jelly-like vitreous humour removed, the retina will be seen and easily separated as a pulpy membrane from the dark coloured choroid on which it rests. But it does not separate completely. At one spot it is bound down. This spot is the entrance of the optic nerve. The nerve (M) comes from the brain (p. 95) and pierces the eyeball at the back, not quite at the middle, but about nth of an inch to the inner side, the nose side. The fibres of the nerve are distributed in the retina. The retina does not extend quite to the front limits of the posterior chamber, but stops short, in a scalloped border, the ora serrata, a little way behind the ciliary processes.
Though the retina is extremely delicate, its structure is very complicated. If a piece of the retina, representing its whole thickness, is ex amined under a microscope it shows a structure exhibited in detail in Fig. 175. The part resting on the choroid coat consists of six-sided gran ular nucleated cells filled with colouring matter (Fig. 173). Outside of that is a layer called from the rods and cones. Outside of these a granular layer, and other nuclear bodies, &c., as
represented in Fig. 175. The two layers nearest the surface of the retina, nearest the vitreous humour, therefore, are a layer of nerve-cells and nerve-fibres. In the retina vessels ramify in the region between the inner granular layer and the surface next the vitreous. The vessels are branches of the artery that enters in the sub stance of the optic nerve.
At the entrance of the optic nerve the retina contains no rods or cones. In the retina at a point exactly in the middle of the back wall, therefore directly opposite the centre of the pupil, (about nth of an inch to the outer side of the optic nerve entrance), there is a yellowish spot of an oval shape, the macula lutea, or yellow spot of Soemmering, which exhibits a central depres sion. At this part the retina is very thin, all the layers being very much diminished in thickness, except that of the rods and cones, the layer of nerve-fibres being absent. In the layer of rods and cones marked differences from other parts also exist, for rods are scarce and cones are very close and numerous.
The rods and cones are to be regarded as the peculiar modes of termination of the nervous filaments in the eye, just as the taste buds are Jacob's membrane, containing bodies termed rods and cones. To this succeeds a layer of nuclear bodies developed in fibres continued the modes of termination of the nerve of taste in the tongue (p. 445), just as the touch-bodies are the terminations of the nerves in the skin (p. 443), and just as epithelial cells of a peculiar shape form the terminations of the nerves of smell in the nostrils.