282 THE RETINA, BY MAX SCHULTZE. 



interim. In the percipient layer the rods already begin to 

 fail, even at the outermost edge of the macula lutea, their 

 place being taken by the cones, and they ultimately altogether 

 disappear. The cones, which are closely arranged, become 

 more and more slender towards the border of the fovea cen- 

 tralis, so that they here resemble the rods. Thus it comes to 

 pass that in the fovea a much greater number of cones find room 

 than in any other corresponding area in the vicinity. The 

 thickness of the cone fibres which are attached to the slender 

 cones of the fovea centralis, and traverse the external granule 

 layer, is, however, not much less than that of the thick cones 

 of the more peripheric parts of the retina. Each slender cone 

 of the fovea terminates also in as large a number of primitive 

 nerve fibrils as the thick cones of the periphery. 



The arrangement of the cones at the yellow spot is surpris- 

 ingly regular.* They are disposed in curved lines, which con- 

 verge towards the centre of the yellow spot, and produce the 

 appearance of shagreen, or that presented by the engine turned 

 back of many watches. This arrangement, which on physio- 

 logical grounds had been predicted by Hensen,f is perfectly 

 regular, the cones successively diminishing in diameter from the 

 periphery of the yellow spot to the margin of the fovea ; at this 

 point, however, that is to say, in the fovea itself, the curves 

 are less regular, and the cones in an area of about 0'2 of a 

 millimeter in diameter are all of equal thickness. 



As the thickness of the cones diminishes towards the fovea, 

 their length increases. The outer segments, which at the more 

 peripheric parts of the retina are concealed amongst and are 

 somewhat shorter than the rods, become at the yellow spot, 

 where they gradually supplant them, equal to or even longer 

 than the rods, especially when, as in the fovea, the remaining 

 layers of the retina retreat somewhat towards the vitreous, as 

 though in order to make room for the longer cones.J In one 

 case I found the longest cones above 100 micromillimeters in 



* Max Schultze, Archiv fur Mikroskop. Anatomic, Band ii., Taf. xii. 

 t Vircliow's Archiv, Band xxxv., p. 403. 



J Max Schultze, Archiv fur Mihroskop. Anatomic, Band ii., p. 229, Taf. 

 xiii., fig. 1. 



