Observations on tlie structure of the central fovea of the human eye. 17 



5. As to the thicJcness of the fibres of the fihrous layer. 



In the greater part of the retina the nucleated bodies of the 

 cones are placed close below the limitans externa and are continuous 

 with the inner segments, the two forming, as is well shown in teased 

 preparations of an osmic retina, a continuous structure of uniform 

 diameter, except where the cone tapers externally. From the inner 

 pole of the nucleated cone body the cone-fibre passes through the 

 outer nuclear layer to end within the outer molecular layer. In the 

 macula lutea just outside the central fovea the inner segments of the 

 cones have become more slender while the cone-nuclei retain about 

 the same size as in other parts of the retina and the part of the 

 cone-body which contains the nucleus is bulged out by the latter. The 

 cones, moreover, being not only more slender but also more numerous 

 here, there is no longer room for all their nucleated bodies in one 

 row, hence we find them at first two or three and ultimately five or 

 six deep, but still forming a nucleated layer close to the limitans 

 externa. The rest of what in the remainder of the retina is the outer 

 nuclear layer is occupied by the cone-fibres as they pass to the outer 

 molecular layer, as well as by the few rod-fibres which are still 

 present. Since some of the nucleated cone bodies are thus removed 

 from the limitans externa it necessarily follows that the part of the 

 cone-cell between the nucleus and the cone -proper is elongated in 

 proportion to the extent of removal of the cone nucleus from the 

 limitans externa. The outer part of the cone-element, which extends 

 from the cone -nucleus to the external limiting membrane, although 

 it may now be di^awn out more or less to the appearance of a fibre 

 is really in direct continuity with and, except for the line formed by 

 the limitans, histologically indistinguishable from the inner segment 

 of the cone proper, with which it forms a continuous structure. The 

 same considerations apply exactly to the cone-elements in the central 

 fovea so that we may describe these as possessing as we approach 

 the centre always longer and longer peripheral portions and shorter 

 and shorter proximal portions; until at the very centre the nucleated 

 bodies are almost sessile upon the external molecular layer, owing 

 to the shortness or practical absence of the proximal portions, and the 



Internationale Monatsschrift für Anat. u. Phys. XII. 2 



