The crystalline lens, hardened and divided.
Ora serrata is the boundary between optic and blind parts, which accords with transition of choroid into ciliary body. In posterior part of retina is found a disc of the optic nerve that has a small concavity. Macula is located in the centre of retina. Central fossa is the place of best sight sharpness, where is observed most rods and cones.
Anterior chamber of eyeball placed between posterior surface of cornea surface and anterior surface of the iris. Posterior chamber is found between posterior surface by iris and anterior surface of lens. The anterior and posterior chambers are filled by aqueous humor, which produced by ciliary processes of ciliary body and unite each other by the medium of pupil. Between cornea and iris is found iridocorneal corner, which is filled by pectinate ligament with the Fontana`s spaces. Aqueous humor draines from anterior chamber through fountain spaces to the Schlemm`s canal (venous sinus of sclera).
The bulb of the eye (bulbus oculi; eyeball), or organ of sight, is contained in the cavity of the orbit, where it is protected from injury and moved by the ocular muscles. Associated with it are certain accessory structures, viz., the muscles, fasciæ, eyebrows, eyelids, conjunctiva, and lacrimal apparatus.
The bulb of the eye is imbedded in the fat of the orbit, but is separated from it by a thin membranous sac, the fascia bulbi (page 1024). It is composed of segments of two spheres of different sizes. The anterior segment is one of a small sphere; it is transparent, and forms about one-sixth of the bulb. It is more prominent than the posterior segment, which is one of a larger sphere, and is opaque, and forms about five-sixths of the bulb. The term anterior pole is applied to the central point of the anterior curvature of the bulb, and that of posterior pole to the central point of its posterior curvature; a line joining the two poles forms the optic axis. The axes of the two bulbs are nearly parallel, and therefore do not correspond to the axes of the orbits, which are directed forward and lateralward. The optic nerves follow the direction of the axes of the orbits, and are therefore not parallel; each enters its eyeball 3 mm. to the nasal side and a little below the level of the posterior pole. The bulb measures rather more in its transverse and antero-posterior diameters than in its vertical diameter, the former amounting to about 24 mm., the latter to about 23.5 mm.; in the female all three diameters are rather less than in the male; its antero-posterior diameter at birth is about 17.5 mm., and at puberty from 20 to 21 mm.
Development.—The eyes begin to develop as a pair of diverticula from the lateral aspects of the forebrain. These diverticula make their appearance before the closure of the anterior end of the neural tube; after the closure of the tube they are known as the optic vesicles. They project toward the sides of the head, and the peripheral part of each expands to form a hollow bulb, while the proximal part remains narrow and constitutes the optic stalk. The ectoderm overlying the bulb becomes thickened, invaginated, and finally severed from the ectodermal covering of the head as a vesicle of cells, the lens vesicle, which constitutes the rudiment of the crystalline lens. The outer wall of the bulb becomes thickened and invaginated, and the bulb is thus converted into a cup, the optic cup, consisting of two strata of cells. These two strata are continuous with each other at the cup margin, which ultimately overlaps the front of the lens and reaches as far forward as the future aperture of the pupil. The invagination is not limited to the outer wall of the bulb, but involves also its postero-inferior surface and extends in the form of a groove for some distance along the optic stalk, so that, for a time, a gap or fissure, the choroidal fissure, exists in the lower part of the cup (865). Through the groove and fissure the mesoderm extends into the optic stalk and cup, and in this mesoderm a bloodvessel is developed; during the seventh week the groove and fissure are closed and the vessel forms the central artery of the retina. Sometimes the choroidal fissure persists, and when this occurs the choroid and iris in the region of the fissure remain undeveloped, giving rise to the condition known as coloboma of the choroid or iris.
The retina is developed from the optic cup. The outer stratum of the cup persists as a single layer of cells which assume a columnar shape, acquire pigment, and form the pigmented layer of the retina; the pigment first appears in the cells near the edge of the cup. The cells of the inner stratum proliferate and form a layer of considerable thickness from which the nervous elements and the sustentacular fibers of the retina, together with a portion of the vitreous body, are developed. In that portion of the cup which overlaps the lens the inner stratum is not differentiated into nervous elements, but forms a layer of columnar cells which is applied to the pigmented layer, and these two strata form the pars ciliaris and pars iridica retinæ.
The cells of the inner or retinal layer of the optic cup become differentiated into spongioblasts and germinal cells, and the latter by their subdivisions give rise to neuroblasts. From the spongioblasts the sustentacular fibers of Müller, the outer and inner limiting membranes, together with the groundwork of the molecular layers of the retina are formed. The neuroblasts become arranged to form the ganglionic and nuclear layers. The layer of rods and cones is first developed in the central part of the optic cup, and from there gradually extends toward the cup margin. All the layers of the retina are completed by the eighth month of fetal life.
The optic stalk is converted into the optic nerve by the obliteration of its cavity and the growth of nerve fibers into it. Most of these fibers are centripetal, and grow backward into the optic stalk from the nerve cells of the retina, but a few extend in the opposite direction and are derived from nerve cells in the brain. The fibers of the optic nerve receive their medullary sheaths about the tenth week after birth. The optic chiasma is formed by the meeting and partial decussation of the fibers of the two optic nerves. Behind the chiasma the fibers grow backward as the optic tracts to the thalami and mid-brain.Поділитися цим: