3 i6 



ELEMENTS OF -HISTOLOGY. [Chap, xxxix. 



layer are limited to a very few layers (generally about 

 two) next to the membrana limitans externa. For 

 this reason, the rest of the outer nuclear layer is 

 occupied by the cone-fibres only, which in the fovea 

 ceiitralis pass in a slanting, or almost horizontal, 



Fig. 161. From a Vertical Section through the Macula Lutea arid 

 Fovea Ceiitralis. 



, Nerve-fibres; 6, ganglion cells: c, inner molecular layer; a, imu-r nuclei; 

 e, outer molecular layer ; /, cone-fibres ; g, cones. (Diagram by Max ScbulUe.) 



direction sideways into the outer molecular layer. 

 The ganglion cells form several strata in the macula 

 lutea. In the fovea ceiitralis are present the cones 

 (very long and thin), the limitans externa, the few 

 nuclei representing the outer nuclear layer, a thin 

 continuation of the inner molecular layer, and the 

 limitans interna. 



448. In the embryo, the primary optic vesicle 

 becomes invaginated so as to form the optic cup, 

 which consists of two layers an outer, giving origin 

 to the pigmented epithelium ; and an inner, the retina 



