ANATOMY OF THE EYEBALL. 



59 



FIGS. 



cells, like those in the brain, measuring from -g-gVfr to 

 of an inch in diameter. In the centre of the retina, at the 

 macula lutea, the cells present eight lay- 

 ers, and they diminish to a single layer 

 near the periphery. The smaller cells are 

 situated near the centre, and the larger, 

 near the periphery. Each cell sends off 

 several filaments (from two to twenty- 

 five) probably going to the layer of rods 

 and cones, and a single filament, which 

 becomes continuous with one of the fila- 

 ments of the optic nerve. 



The layer formed by the expansion 

 of the optic nerve is composed of pale, 

 transparent nerve-fibres, from go ^ 00 to 

 2g i 6Tr of an inch in diameter. These 

 do not call for special description. 



The limitary membrane is a delicate 

 structure, with fine striae and nuclei, com- 

 posed of connective-tissue elements. It 

 is about Tg^-J-jny of an inch in thickness. 

 From this membrane, connective-tissue 

 elements are sent into the various layers 

 of the retina, and form a framework for 

 the support of the other structures. 1 



As before remarked, the retina be- 

 comes progressively thinner from the 

 centre to the periphery. The granular 

 layers and the nervous layers are rapidly 

 lost in the anterior half of the membrane. 



The connection between the rods 

 and cones and the ganglion-cells may be 



Perpendicular section of the 

 human retina, made six 

 lines anterior to the en- 

 trance of the optic nerve, 

 magnified 350 times. 1, 

 bacillar layer ; 2, external 

 granule -layer; 3, inter- 

 granule layer (cone-fibre 

 plexus) ; 4." internal gran- 

 ule-layer; 5, finely gran- 

 ular gray layer; 6, layer 

 of nerve-cells; 7, fibres 

 of the optic nerve; 8, 

 the membrana limitans. 

 (K6LL1KER, Manual of 

 Hum. a n Microscopic 

 Anatomy, London, I860, 

 p. 553.) 



1 It is only of late years that the connective-tissue elements of the retina 

 have been accurately recognized. Formerly, much confusion existed because 

 these elements were confounded with the elements of true nerve-tissue. The 

 fibres of Miiller undoubtedly belong to connective tissue. 



