THE ORGAN OF SIGHT 



1001 



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 



Cavity of fore-brain 



Imagination of 



ectoderm to form 



lens rudiment 



Pigmented layer of retina 



Margin of optic cup 



Nervous layer of retina 

 Optic vesicle 



FIG. 863. Transverse section of head of chick embryo of forty-eight hours' incubation. (Duval.) 



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 diame- 

 ters 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 fore- 

 brain. 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 re- 

 mains narrow and constitutes the 

 optic stalk (Figs. 863, 864). The 

 ectoderm overlying the bulb be- 

 comes thickened, invaginated, and finally severed from the ectodermal covering 

 of the head as a vesicle of cells, the lens vesicle, which constitutes the rudi- 

 ment 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 



Cavity of fore-brain 



Pigmented layer 

 of retina 



Ectoderm 



Lens 



Nervous layer of 

 retina 



Optic stalk 



FIG. 864. Transverse section of head of chick embryo of 

 fifty-two hours' incubation. (Duval.) 



