RETINA. 



189 



Retina. 



The fibres of the optic nerve, which pass into the eyeball 

 at a right or acute angle, cross one another at the point of 

 entrance, 1 and are then distributed to the sensitive elements of 

 the retina. 



The latter is thus thickest at the point of entrance of the 

 nerve, which is known as the "blind-" or "Marriott's spot," and 



FIG. 15(5. RETINA. (After Merkel.) (The nervous portion is shown black, and 

 the supporting substance of a lighter tint. ) 



O, layer of nerve-fibres; GgL, layer of nerve-cells; in.gr, inner granular layer; 

 in. K, inner nuclear layer; au.gr, outer granular layer; duK, outer nuclear 

 layer ; Lc, limitans externa ; St, layer of rods and cones. 



gradually decreases in thickness towards the ciliary processes, 

 until, at the point of origin of the iris, it consists of a simple layer 

 of cells. 



The retina, which is bounded both on its inner and outer 

 periphery 2 by a structureless hyaline membrane (limitans 



1 This has not been satisfactorily made out in Mammals. 



2 TLe limitans externa encloses the entire retina externally in the embryo, but 

 later the rods and cones come to project through it (see Fig. 156). 



