THE ORGANS OF SPECIAL SENSE. 587 



the nerve fibers begin to grow into the optic stalk, the fibers must pass through 

 these two coats in their exit from the eye. There results the fenestrated cross- 

 ing of the optic nerve by these two coats, known as the lamina cribrosa. 



The optic nerve fibers are medullated but have no neurilemmas. They are 

 supported by neuroglia. The connective tissue sheaths which enclose the optic 

 nerve are direct extensions of the meninges. These structural peculiarities 

 accord with the peculiarities already described in the development of the 

 nerve. Attention has been called to the fact (p. 573) that just as the retina 

 should be considered a modified and displaced portion of the central nervous 

 system— of brain cortex — so the optic nerve should be considered not as a 

 peripheral nerve, but as analogous to a central nervous system fiber tract. 



The Ciliary Body, Iris, Cornea, Anterior Chamber. — Anteriorly where 

 they come into relation with the lens and are so arranged as to admit light to the 

 retina, all three coats of the eye are extensively modified. Thus the retina is 

 continued anteriorly as the pars ciliaris retinae and pars iridica retinas, the 

 chorioid as the stroma of the ciliary body and iris, the sclera as the cornea. 



The Ciliary Body and Iris. — Both primary retinal layers (the two layers 

 of the optic cup) are continued anteriorly as the non-nervous retinal layer 

 of the ciliary body and iris. The outer pigmented layer consists at first of 

 several layers of pigmented cells, but later becomes reduced to a single layer 

 of pigmented cells which do not, however, possess pigmented processes extend- 

 ing inward as do the analogous cells of the nervous retina. The abrupt tran- 

 sition at the ora serrata where the thick pars optica retinae passes over into the 

 pars ciliaris retinas has been mentioned (p. 580). The inner layer of the primi- 

 tive retina (optic cup) extends over the ciliary body and iris as a single layer of 

 cells. These remain non-pigmented over the ciliary body, but over the iris 

 acquire pigment so that the two layers form the pigmented layer of the iris. 



The mesodermic tissue which forms the stroma of the ciliary body and iris 

 is derived from the mesoderm lying between the lens and the surface ectoderm. 

 This separates into two layers enclosing between them the anterior chamber of 

 the eye, and it is from the posterior of these two layers that mesodermic tissue 

 extends into the ciliary body and iris. It is continuous with the mesoderm of 

 the tunica vasculosa lentis. During the fourth month the ciliary body under- 

 goes foldings to form the ciliary processes. These foldings at first involve 

 also the iris, but the iris folds soon (end of fifth month) disappear, while the 

 ciliary processes become more prominent. 



Of the smooth muscle tissue found in the ciliary body and iris, the dilator 

 and contractor pupillae are, according to Bonnet, derived from the cells of the 

 pigmented layer of the retina, i.e., from ectoderm. The ciliary muscle, on the 

 other hand, develops from mesoderm. These muscles become well developed 

 during the seventh month. 



