LESSON IX. THE ORGAN OF SIGHT. 1. IN studying the organ of the sense of sight, the eye, it is needful to become acquainted, firstly, with the structure and properties of the sensory expansion in which the optic nerve terminates; secondly, with the physical agent of the sensation; thirdly, with the intermediate apparatus by which the physical agent is enabled to act upon the nervous expansion. The ball of the eye is a globular body, moving freely in a chamber, the orbit, which is furnished to it by the skull. The optic nerve, the root of which is in the brain, leaves the skull by a hole at the back of the orbit, and enters the back of the globe of the eye, not in the middle, but on the inner, or nasal, side of the centre. It then spreads out on the inner surface of the wall of the globe of the eye, and enters a very delicate membrane, which extends forward nearly to the margin of the crystalline lens, varying in thickness from th of an inch to less than half that amount, and is termed the retina. This retina is the only organ connected with sensory nervous fibres which can be affected, by any agent, in such a manner as to give rise to the sensation of light. 2. If the globe of the eye be cut in two, transversely, so as to divide it into an anterior and a Diagrammatic views of the nervous (A) and the connective (B) elements of the retina, supposed to be separated from one another. A, the nervous structures-b, the rods; c, the cones; 'c' the granules of the outer layer, with which these are connected; d d', interwoven very delicate nervous fibres, from which fine nervous filaments, bearing the inner granules, ff', proceed towards the front surface; g g', the continuation of these fine nerves, which become convoluted and interwoven with the processes of the ganglionic corpuscles, h h'; i i, the expansion of the fibres of the optic nerve B, the connective tissue-a a, external or posterior limiting membrane; e e, radial fibres passing to the internal or anterior limiting membrane; e' e', nuclei; d d, the intergranular layer; g g, the molecular layer; 1, the anterior limiting membrane. Magnified about 250 diameters. posterior half, the retina will be seen lining the concave wall of the posterior half as a membrane of great delicacy, and, for the most part, of even texture and smooth surface. But, exactly opposite the middle of the posterior wall, it presents a slight circular depression of a yellowish hue, the macula lutea, or yellow spot; and, at some distance from this, towards the inner, or nasal, side of the ball, is a radiating appearance, produced by the entrance of the optic nerve and the spreading out of its fibres into the retina. 3. A very thin vertical slice of the retina, in any region except the yellow spot and the entrance of the optic nerve, may be resolved into the structures represented separately in Fig. 63. The one of these (A) occupies the whole thickness of the section, and comprises its essential, or nervous, elements. The outer (or posterior) fourth, or rather less, of the thickness of these consists of a vast multitude of minute, rod-like, and conical bodies, ranged side by side, perpendicularly to the plane of the retina. This is the layer of rods and cones (bc). From the front ends of the rods and cones very delicate fibres pass, and in each is developed a granule-like body (b' c'), which forms a part of what has been termed the outer layer of granules. It is probable that these fibres pass into and indeed form the close meshwork of very delicate nervous fibres which is seen at d d' (Fig. 63, A). From the anterior surface of this meshwork other fibres proceed, containing a second set of granules, which forms the inner granular layer (ff'). In front of this layer is a stratum of convoluted fine nervous fibres (g g')—and anterior to this again numerous ganglionic corpuscles (h h'). Processes of these R ganglionic corpuscles extend, on the one hand, into the layer of convoluted nerve-fibres; and on the other are probably continuous with the stratum of fibres of the optic nerve (i). These delicate nervous structures are supported by a sort of framework of connective tissue, which extends from an inner or anterior limiting membrane (1), which bounds the retina and is in contact with the vitreous humour, to an outer or posterior limiting membrane, which lies at the anterior ends of the rods and cones near the level of b'c' in Fig. 63. Thus the framework is thinner than the nervous substance of the retina, and does not extend between the rods and cones, which lie between it and the pigment of the choroid coat (§ 16). The fibres of the optic nerve spread out between the limiting membrane (7) and the ganglionic corpuscles (h'); and the vessels which enter along with the optic nerve ramify between the limiting membrane and the inner granules (ƒ ƒ'). Thus, not only the nervous fibres, but the vessels, are placed altogether in front of the rods and cones. At the entrance of the optic nerve itself, the nervous fibres predominate, and the rods and cones are absent. In the yellow spot, on the contrary, the cones are abundant and close set, becoming at the same time longer and more slender, while rods are scanty, and are found only towards its margin. The layer of fibres of the optic nerve disappears, and all the other layers, except that of the cones, become extremely thin in the centre of the macula lutea (Fig. 64). 4. The most notable property of the retina is its power of converting the vibrations of ether, which constitute the physical basis of light, into a stimulus to the fibres of the optic nerve-which fibres, when |