354 HISTOLOGY. 



the central artery and vein of the retina, around which they pass out of the 

 optic cup (Fig. 409, D). They grow beneath and among the cells of the 

 optic stalk to the brain, which they enter. These fibers which constitute 

 the optic nerve, cause the obliteration of the optic stalk. It is shown in the 

 figure that the optic nerve at its origin interrupts the retinal layers, pro- 

 ducing a "blind spot." The part of the nerve which forms the blind 

 spot, with the vessels in the center, is called the papilla of the optic 

 nerve. 



The lens (Fig. 409, D) loses its central cavity by the elongation of the 

 cells in its posterior layer. These become the fibers of the lens. The 

 anterior layer remains throughout life as a simple epithelium, called the 

 epithelium of the lens (Fig. 410). The lens becomes covered by an 

 elastic capsula lentis and in embryonic life it possesses a vascular capsule 

 (Fig. 409, E) containing branches of the central artery. The vascular 

 layer covering the anterior surface of the lens is designated the pupillary 

 membrane, and it disappears shortly before birth. Its occasional persistence 

 interferes with vision. 



Between the lens and the retina there is a peculiar tissue, mucoid in 

 appearance and resembling mesenchyma in form. Since processes from 

 the retina and from the lens have been found extending into it, it is con- 

 sidered to be essentially ectodermal. Its blood vessels become obliterated 

 and it forms the vitreous body of the adult, consisting of a stroma and a 

 humor. Extending through it, from the papilla of the optic nerve toward 

 the lens, is the hyaloid canal, which in the embryo lodged the hyaloid artery 

 (a prolongation of the central artery). Sometimes this artery is repre- 

 sented in the adult by a strand of tissue. The vitreous body is surrounded 

 by a fibrous layer called the hyaloid membrane. 



A cavity forms in the tissue in front of the lens and becomes filled with 

 a watery tissue fluid (aqueous humor). It is bounded by a mesenchymal 

 epithelium. The portion of the cavity which is anterior to the retinal cup 

 and lens is called the anterior chamber of the eye; the smaller part within the 

 retinal cup but in front of the lens and the fibrous covering of the vitreous 

 body, is the posterior chamber (Fig. 309, E, c.p.). 



The retinal cup is surrounded by two layers of mesenchymal origin. 

 The inner tunica vasculosa corresponds with the pia mater and forms the 

 chorioid coat of the eye; the outer tunica fibrosa corresponds with the dura 

 and forms the sclera, into which the muscles of the eye are inserted. The 

 portion of the retinal cup which forms a curtain, circular in front view, 

 between the anterior and posterior chambers, is called the iris. It consists 

 of the tunica vasculosa together with a thin pigmented prolongation of the 

 retina over its posterior surface (Fig. 410). This pars iridica retinae is 



