RETINA. 



18'J 



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. 150. RETINA. (After Merkel.) (The nervous portion is shown black, and 

 the supporting substance of a lighter tint.) 



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

 in.K, inner nuclear layer; du.gr, outer granular layer; aiiK, outer nuclear 

 layer ; Le, 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. 



TLe limitars externa encloses the entire retina externally in the embryo, but 

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



