532 



THE NERVOUS SYSTEM. 



peculiarities of structure corresponding with its physiological endow- 

 ments. 



The macula lutea is distinguished, in the first place, by the absence 

 of the layer of nerve fibres. Those fibres, according to Kolliker, 

 which, in radiating from the entrance of the optic nerve, reach the 

 edges of the macula, lose themselves among the nerve cells of its 

 gano-lionic layer. The others curve round its borders on each side, 

 and resume their peripheral course beyond ; so that there are none 

 to be found within the limits of the yellow spot. 



Secondly, the nerve cells of the ganglionic layer are more abundant 

 in the macula lutea than elsewhere. Over the greater portion of the 

 retina, according to Schultze, these cells are arranged in a single plane ; 



FIG. 138. 





m2 



DIAGRAMMATIC SECTION OF HUMAN RETINA, through the macula lutea and fovea centralis. 1. 

 Inner surface of the retina. 2. Ganglionic layer of nerve cells. 3. Intermediate layers of the 

 retina, disappearing at the centre of the macula lutea. 4. Layer of nuclei, showing the oblique 

 course of its fibres in this region. 5. Layer of rods and cones ; consisting at its central portion, 

 exclusively of attenuated and elongated cones. 6. Outer surface of the retina. The depression 

 in the middle of the diagram is the fovea centralis. (Schultze.) 



but in the yellow spot they form several superimposed ranges. Toward 

 the centre of the yellow spot, on the other hand, they diminish in num- 

 ber, and are entirely wanting at the fovea centralis. 



Thirdly, owing to these modifications, the retina, at the fovea cen- 

 tralis, consists only of its two external layers, namely, the layer of nuclei 

 and the layer of rods and cones. Even these layers exhibit, at the 

 same point, important peculiarities in the form and arrangement of their 

 elements. 



In the layer of nuclei, the fibres with which they are connected, instead 

 of remaining perpendicular to the surface of the retina, bend obliquely 

 outward, to reach its more superficial parts in the outer portions, or 

 beyond the borders, of the yellow spot. Thus the layer is much 

 diminished in thickness, though still containing nuclei, connected, by 

 their usual extensions, with other parts of the retina. 



Finally, the layer of rods and cones, at the macula lutea and fovea, 

 is distinguished by special features from the corresponding parts else- 

 where. At this situation it is increased in thickness, and consists 

 exclusively of slender elongated cones. The diameter of the cones at 



