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 re- 

 places the usual single row. 



The fovea, on the other hand, 

 is produced by the hollowing out 

 of the centre of the macula conse- 

 quent upon the gradual thinning 

 and final suspension of the reti- 

 nal layers lying internal to the 

 outer nuclear zone ; the centre 

 of the foveal depression, the fun- 

 dus fovese, thus is composed of 

 the neuro - epithelial and the 

 pigment layer alone. Within 

 a central area, about .5 mm. in 

 diameter, the fovea is also 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 diminu- 

 tion in the thickness of the retina, 

 brought about by the sudden ter- 

 mination at this point of many of 

 its layers. The regular diminu- 

 tion in the retinal thickness pro- 

 ceeds gradually from the fundus 

 towards the periphery, when, on 

 reaching a point near the ora ser- 

 rata, many layers end abruptly, 

 the ciliary continuation measuring 

 only about one-third of the thick- 

 ness OI the adjacent retina. Diagrammatic section of human retina through 



macula and fovea: 2, fibre-layer; i, ganglion- 

 cells ; 4, 5, inner reticular and inner granule layer ; 6, outer reticular layer followed by obliquely-disposed 

 elements of outer nuclear layer (7, -ja) ; 9, rods and cones ; 10, pigment. (After Max Schultze.) 



