Henry Gray (18251861). Anatomy of the Human Body. 1918.
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.
FIG. 863 Transverse section of head of chick embryo of forty-eight hours incubation. (Duval.) (See enlarged image)
FIG. 864 Transverse section of head of chick embryo of fifty-two hours incubation. (Duval.) (See enlarged image)
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 (Figs. 863,864). 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 (Fig. 864). 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