836 



STRUCTURE OF THE EYE. 



[Book hi. 



cup becomes what we may speak of broadly as the retina, and 

 we may call it the optic or retinal cup ; the solid stalk becomes 

 the optic nerve ; and the other parts of the eyeball are formed 

 round this retinal cup, which remains as the essential part of 

 the eye. 



Fig. 139. Diagrammatic outline of a horizontal section of the eye, 

 to illustrate the relations of the various parts. 



The figure is to be regarded as very diagrammatic, more or less distortion of 

 the relative sizes of the various parts and of the relative thickness of the coats 

 being unavoidable in the effort to secure simplicity. 



Scl. the sclerotic coat, shaded longitudinally, continuous with the (unshaded) 

 body of the cornea, ex. the epithelium of the cornea continuous with e.cj. the 

 epithelium of the conjunctiva. 



Ch. the choroid coat with C. P. the ciliary process and /. the body of the iris, 

 all stippled to indicate that they are all parts of the same vascular investment. 



R. the retina or inner wall, and P. E. the pigment epithelium or outer wall of 

 the retinal cup. In front of the wavy line OS., marking the position of the ora 

 serrata, the retina proper changes into the pars ciliaris retinae, p. c. B- Both the 

 pigment epithelium and the pars ciliaris retinae are represented as continued over 

 the back of the iris as well as over the ciliary process. 



L. the lens. sp. I. the suspensory ligament. The broken line round the lens, 

 shewn on one side only, represents the membrana capsulo-pupillaris ; and the 

 straight continuation of it through V. H. the vitreous humour to O. N. the optic 

 nerve indicates the embryonic continuation of the central artery of the retina. 



o. x. the optic axis, in this case made to pass through the fovea centralis fc. 



The front or inner wall of the retinal cup is from the first 

 distinctly thicker than the hind or outer wall (Fig. 139) ; it soon 

 consists of more than one layer of epithelium, and it alone, or, 

 more strictly speaking, part of it alone, becomes the retina proper. 

 The hind or outer wall remains thin, and continues to consist of a 

 single layer of epithelium, the cells of which are never developed 



