The back portion of the globe of the eye is covered with a curtain, the retina, formed by the spreading out of the fibres of the optic nerve in front of various layers of nerve-cells and the sensory cells of the organ of vision, rods and cones. The retina lies between the hyaloid membrane, which encloses the vitreous humour, and a layer of pigment which “backs” it, as a photographer backs a plate when he proposes to use it towards a source of light—to take a photograph of a window from within a room. The serrated margin of the retina is somewhat anterior to the equator of the eyeball. The pigment which backs the retina is contained in a sheet of cells which belongs to the pouch of brain that extended outwards towards the eye-pit ([p. 334]). Properly speaking, therefore, it is a layer of the retina.
Fig. 28.—Diagrams showing the Mode of Formation of the Crystalline Lens.
A, A pit in the epithelium on the surface of the head has closed into a hollow sphere. B, The cells of the posterior wall of this sphere are growing forward, as the fibres of the lens which traverse its whole thickness, with the exception of the cubical epithelium on its front.
Three sets of tissues take part in the development of the eyeball. (1) The epithelium covering the surface of the head is depressed as a pit, which gradually closes into a hollow sphere. This sphere, when its cavity is filled up, owing to the great elongation of the cells of its posterior half, becomes the lens. It breaks away from the rest of the epithelium of the surface, which clears to transparency as that part of the conjunctiva termed the “corneal epithelium.” (2) The retina, as already stated, is a hollow outgrowth from the interbrain. As this pouch approaches the lens, its anterior half is pushed back into the posterior half, forming a cup with a double wall. The anterior, or inner, sheet of the bowl of the cup develops into the nervous layers of the retina, the posterior sheet into its pigmented epithelium. (3) Connective tissues are transformed into the other constituents of the globe—cornea, iris, vitreous humour, etc. The globe is complete, except at a spot on the nasal side of its posterior pole where the optic nerve pierces it.
The bloodvessels of the retina, entering with the optic nerve, ramify on its anterior surface. Under ordinary circumstances we ignore the shadows which they cast, as we ignore the blind spot which coincides with the disc of insensitive tissue presented by the end of the optic nerve, and many other imperfections; but it was shown by Purkinje many years ago that by a very simple manœuvre they may be forced upon our notice.
Fig. 29.—Purkinje’s Shadows.