242 NORMAL HISTOLOGY. 



THE RETINA. 



Of all the animal structures the retina is one of 

 the most delicate, complicated, and difficult of 

 study, and we can do little more here than indicate 

 briefly the general grouping of its elements. It 

 consists of a connective-tissue framework by which 

 the nerve-elements are supported, and with which 

 they are most intimately associated ; and in some 

 cases it is as yet impossible to say to which variety 

 of tissue a given element belongs. We distinguish, 

 in typical parts, ten layers ; commencing from 

 within, they may be enumerated as follows: 



1. Membrana limitans interna. 



2. Layer of nerve-fibres. 



3. Layer of ganglion-cells. 



4. Internal molecular layer. 



5. Internal nuclear layer. 



6. External molecular layer. 



7. External nuclear layer. 



8. Membrana limitans externa. 



9. Layer of rods and cones. 



10. Pigment layer. 



The limiting membranes, the outer of which is 

 perforated by numerous openings, are very delicate 

 and homogeneous, and belong to the connective- 

 tissue framework. In the layer of nerve-fibres, 

 which is thickest around the entrance of the optic 

 nerve, thus forming the papilla, the fibres spread 

 out, intricately interlacing, into a thin sheet, and 



