838 STRUCTURE OF THE EYE. [BOOK in. 



said thick, and is developed into the retina ; but this takes place 

 only over about the hind three-fourths of the cup. Along a 

 meridian round the eye, at a wavy boundary line called the ora 

 serrata (Fig. 139 #.$.), the retina proper ceases and the inner wall 

 of the retinal cup in front of the ora serrata is continued on as 

 a much thinner structure (Fig. 139 p,c.R.~) consisting of a single 

 layer only of cells ; this is spoken of as the pars ciliaris retince. 

 The outer or hind wall of the retinal cup consists throughout 

 of a single layer of epithelium cells loaded with pigment. Behind 

 the ora serrata, that is, in the region of the retina proper, these 

 cells have, as we shall see, peculiar features, but in front of the 

 ora serrata they lose these features and become ordinary cubical 

 cells, though still loaded with pigment. 



Hence the choroid may be described as having a double lining. 

 Over the hind part of the eye, behind the ora serrata, it is lined 

 by the single layer of pigment epithelium and the retina. In front 

 of the ora serrata it, including the ciliary processes, is lined by 

 the same layer of pigment epithelium representing the outer 

 wall, and by the single layer of cells, free from pigment, repre- 

 senting the inner wall of the retinal cup, the latter being called, 

 as we have said, the pars ciliaris retinae. And as the ciliary part 

 of the choroid passes on to form the iris, these two layers are 

 also continued on to line the back of the iris, coming to an end 

 at the margin of the pupil or central opening of the iris, which 

 may accordingly be taken as marking the extreme lip of the 

 retinal cup. Fig. 139. Here however, as we shall see, the two 

 layers are not so easily and distinctly recognized as in the ciliary 

 region ; and the nature of the structures forming the back of the 

 iris has been a matter of much controversy. 



At an early stage the mesoblastic tissue, which fills up the 

 hollow of the retinal cup and surrounds the lens, is continuous 

 at the mouth of the retinal cup with the outer investment of the 

 cup ; it here forms around the lens the membrana capsulo-pupil- 

 laris, and at the margin of the iris the membrana pupillaris block- 

 ing up the future opening of the pupil. The arteria centralis 

 retinae, which during the folding of the cup and stalk is carried 

 into the core of the optic nerve, does not at this early stage stop 

 at the retina, but is continued forward through the middle of the 

 vitreous humour to the membrana capsulo-pupillaris, and furnishes 

 the developing lens with an abundant supply of blood. But 

 neither layer of the retinal cup stretches over the pupillary 

 membrane ; they both stop, as we have said, at the margin of the 

 iris. Before birth takes place, the membrana pupillaris is, in man, 

 absorbed and the pupil is thus established ; at the same time the 

 central artery in the vitreous humour is obliterated beyond the 

 retina, and the vascular membrana capsulo-pupillaris gives place 

 to the non-vascular capsule of the lens and the suspensory liga- 

 ment of which we shall speak hereafter. 



