CHAP. IIL] SIGHT. 1143 



coats is developed a small mass of muscular fibres, the ciliary 

 muscle of which we shall speak in detail presently. 



The inner or front wall of the retinal cup becomes as we have 

 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. 134 O.S.) , 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. 134 p.c.R.) consisting of a single 

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

 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. 134. Here however, as we shall see, the two 

 layers are not so easily and distinctly recognised 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-pupillaris, 

 and at the margin of the iris the membrana pupillaris blocking 

 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 



