THE EYE AND ITS APPENDAGES. 



357 



The increased thickness of the retina at the macular margin 

 depends almost entirely upon the extraordinary development of 

 the layer of ganglion-cells, which progresses until a stratum from 

 seven to nine cells deep replaces the usual single row. 



The fovea, on the other hand, is produced by the hollowing out 

 of the centre of the macula consequent upon the gradual thinning, 

 to almost suspension, of the retinal layers lying internal to the 



External surface 



FIG. 378. 



M 



Diagrammatic section of the human fovea. Magnified 375 diameters. (Golding-Bird and 

 Schafer.) 2, ganglion layer; 4, inner nuclear layer; 6, outer nuclear layer, the cone-fibres forming 

 the so-called external fibrous layer of Henle ; 7, cones ; z>, section of a blood-vessel ; M, membrana 

 limitans externa ; og, ig, outer and inner granules (cone-nuclei and bipolars) at the centre. 



outer nuclear zone ; the centre of the foveal depression, the fun- 

 dus foveae, consists chiefly of the neuro-epithelium. Within a 

 central area the fovea is devoid of blood-vessels. 



The ora serrata marks the termination of the optical part 

 of the retina and the transition into its anterior continuations, the 

 pars ciliaris and the pars iridica. The ora is distinguished, in 

 addition to its irregular serrated border, by the abrupt diminution 

 in the thickness of the retina, brought about by the sudden termina- 

 tion at this point of many of its layers. The regular diminution in 

 the retinal thickness proceeds gradually from the fundus towards 

 the periphery, when, on reaching a point near the ora serrata, many 

 layers end abruptly, the ciliary continuation measuring only about 

 one-third of the thickness of the adjacent retina. 



