442 HISTOLOGY 



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

 sidered to be essentially ectodermal. Its blood vessels become obliter- 

 ated 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 

 represented 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 mesen- 

 chymal 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. 451, 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 mater 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 tunica vasculosa with a thin pigmented prolongation 

 of the retina over its posterior surface (Figs. 451, E, and 452). This pars 

 iridica retina is rudimentary and without visual function. The iris is 

 covered by the mesenchymal epithelium of the chambers. At the at- 

 tached border of the iris the vascular coat contains important muscle 

 fibers, and is there thickened to form the ciliary body. This is also covered 

 by a rudimentary pigmented layer on its inner surface, the pars ciliaris 

 retina. At the ora serrata (Fig. 467) an abrupt thickening of the visual 

 layer of the retina marks the boundary between its ciliary and optic 

 portions. The pars optica retina extends from the ora to the optic 

 nerve, covered externally by the chorioid and sclera. 



As a relatively frequent congenital anomaly, the chorioid fissure fails to close 

 normally and the resulting defect is known as coloboma. If the closure has been 

 nearly complete, so that there is merely a notch at the free margin of the optic cup, 

 it appears in the adult as a median ventral cleft in the iris, so that the pupil is shaped 

 like an inverted pear. If the deeper parts of the chorioid fissure fail to unite, there 

 will be a median ventral gap in the optic portion of the retina, which may seriously 

 interfere with vision. 



The cornea is the tissue in front of the anterior chamber, consisting of 

 a non-vascular mesenchymal tissue, bounded posteriorly by mesenchymal 

 epithelium and anteriorly by the epidermal ectoderm. It is extremely 

 transparent. The epidermal ectoderm extends from the cornea and front 

 of the eye over two folds which form the eyelids. They have met in 



