lamina fusca. The cornea is quite continuous with the sclerotic
but has a greater convexity. Under the microscope it is seen to
consist of five layers. Most anteriorly there is a layer of stratified
epithelium, then an anterior elastic layer, then the substantia
propria of the cornea which is fibrous with spaces in which the
stellate corneal corpuscles lie, while behind this is the posterior
elastic layer and then a delicate layer of endothelium. The
transparency of the cornea is due to the fact that all these
structures have the same refractive index.
The middle or vascular coat of the eye consists of the choroid, the ciliary processes and the iris. The choroid (fig. 1, ch) does not come quite as far forward as the corneo-scleral junction: it is composed of numerous blood-vessels and pigment cells bound together by connective tissue and, superficially, is lined by a delicate layer of pigmented connective tissue called the lamina suprachoroidea in contact with the already-mentioned perichoroidal lymph space. On the deep surface of the choroid is a structureless basal lamina.
The ciliary processes are some seventy triangular ridges, radially arranged, with their apices pointing backward (fig. 1, pc), while their bases are level with the corneo-scleral junction. They are as vascular as the rest of the choroid, and contain in their interior the ciliary muscle, which consists of radiating and circular fibres. The radiating fibres (fig. 1, mc) rise, close to the canal of Schlemm, from the margin of the posterior elastic lamina of the cornea, and pass backward and outward into the ciliary processes and anterior part of the choroid, which they pull forward when they contract. The circular fibres lie just internal to these and are few or wanting in short-sighted people.
The iris (fig. 1, I) is the coloured diaphragm of the eye, the centre of which is pierced to form the pupil; it is composed of a connective tissue stroma containing blood-vessels, pigment cells and muscle fibres. In front of it is a reflection of the same layer of endothelium which lines the back of the cornea, while behind both it and the ciliary processes is a double layer of epithelium, deeply pigmented, which really belongs to the retina. The pigment in the substance of the iris is variously coloured in different individuals, and is often deposited after birth, so that, in newly-born European children, the colour of the eyes is often slate-blue owing to the black pigment at the back of the iris showing through. White, yellow or reddish-brown pigment is deposited later in the substance of the iris, causing the appearance, with the black pigment behind, of grey, hazel or brown eyes. In blue-eyed people very little interstitial pigment is formed, while in Albinos the posterior pigment is also absent and the blood vessels give the pink coloration. The muscle fibres of the iris are described as circular and radiating, though it is still uncertain whether the latter are really muscular rather than elastic. On to the front of the iris, at its margin, the posterior layer of the posterior elastic lamina is continued as a series of ridges called the ligamentum pectinatum iridis, while between these ridges are depressions known as the spaces of Fontana.
The inner or sensory layer of the wall of the eyeball is the retina; it is a delicate transparent membrane which becomes thinner as the front of the eye is approached. A short distance behind the ciliary processes the nervous part of it stops and forms a scalloped border called the ora serrata, but the pigmented layer is continued on behind the ciliary processes and iris, as has been mentioned, and is known as the pars ciliaris retinae and pars iridica retinae. Under the microscope the posterior part of the retina is seen to consist of eight layers. In its passage from the lens and vitreous the light reaches these layers in the following order:—(1) Layer of nerve fibres; (2) Layer of ganglion cells; (3) Inner molecular layer; (4) Inner nuclear layer; (5) Outer molecular layer; (6) Outer nuclear layer; (7) Layer of rods and cones; (8) Pigmented layer.
The layer of nerve fibres (fig. 2, 2) is composed of the axis-cylinders only of the fibres of the optic nerve which pierce the sclerotic, choroid and all the succeeding layers of the retina to radiate over its surface.
The ganglionic layer (fig. 2, 3) consists of a single stratum of large ganglion cells, each of which is continuous with a fibre of the preceding layer which forms its axon. Each also gives off a number of finer processes (dendrites) which arborize in the next layer.
The inner molecular layer (fig. 2, 4) is formed by the interlacement of the dendrites of the last layer with those of the cells of the inner nuclear layer which comes next.
The inner nuclear layer (fig. 2, 5) contains three different kinds of cells, but the most important and numerous are large bipolar cells, which send one process into the inner molecular layer, as has just been mentioned, and the other into the outer molecular layer, where they arborize with the ends of the rod and cone fibres.
The outer molecular layer (fig. 2, 6) is very narrow and is formed by the arborizations just described. The outer nuclear layer (fig. 2, 7), like the inner, consists of oval cells, which are of two kinds. The rod granules are transversely striped, and are connected externally with the rods, while internally processes pass into the outer molecular layer to end in a knob around which the arborizations of the inner nuclear cells lie. The cone granules are situated more externally, and are in close contact with the cones; internally their processes form a foot-plate in the outer molecular layer from which arborizations extend.
The layer of rods and cones (fig. 2, 9) contains these structures, the rods being more numerous than the cones. The rods are spindle-shaped bodies, of which the inner segment is thicker than the outer. The cones are thicker and shorter than the rods, and resemble Indian clubs, the handles of which are directed outward and are transversely striped. In the outer part of the rods the visual purple or rhodopsin is found.
The pigmented layer consists of a single layer of hexagonal cells containing pigment, which is capable of moving towards the rods and cones when the eye is exposed to light and away from them in the dark.
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| Fig. 2.—Diagrammatic section through the retina to show the several layers, which are numbered as in the text. Ct, The radial fibres of the supporting connective tissue. |
Supporting the delicate nervous structures of the retina are a series of connective tissue rods known as the fibres of Müller (fig. 2, Ct); these run through the thickness of the retina at right angles to its surface, and are joined together on the inner side of the layer of nerve fibres to form the inner limiting membrane. More externally, at the bases of the rods and cones, they unite again to form the outer limiting membrane.
When the retina is looked at with the naked eye from in front two small marks are seen on it. One of these is an oval depression about 3 mm. across, which, owing to the presence of pigment, is of a yellow colour and is known as the yellow spot (macula lutea); it is situated directly in the antero-posterior axis of the eyeball, and at its margin the nerve fibre layer is thinned and the ganglionic layer thickened. At its centre, however, both these layers are wanting, and in the layer of rods and cones only the cones are present. This central part is called the fovea centralis and is the point of acutest vision. The second mark is situated a little below and to the inner side of the yellow spot; it is a circular disk with raised margins and a depressed centre and is called the optic disk; in structure it is a complete contrast to the yellow spot, for all the layers except that of the nerve fibres are wanting, and consequently, as light cannot be appreciated here, it is known as the “blind spot.” It marks the point of entry of the optic nerve, and at its centre the retinal artery appears and divides into branches. An appreciation of the condition of the optic disk is one of the chief objects of the ophthalmoscope.
The crystalline lens (fig. 1, L) with its ligament separates the aqueous from the vitreous chamber of the eye; it is a biconvex lens the posterior surface of which is more curved than the anterior. Radiating from the anterior and posterior poles are three faint lines forming a Y, the posterior Y being erect and the anterior inverted. Running from these figures are a series of lamellae, like the layers of an onion, each of which is made up of a number of fibrils called the lens fibres. On the anterior surface of the lens is a layer of epithelial cells, which, towards the margin or equator, gradually elongate into lens fibres. The whole lens is enclosed in an elastic structureless membrane, and, like the
