THE INTERNAL COAT THE NERVOUS TUNIC 



661 



The much elongated nuclear portion of the cones deviates in a slanting 

 direction toward the margin of the macula, and the cone nuclei are 

 further removed from the external limiting membrane than elsewhere 

 in the retina. 



The pigment of the epithelial layer is much diminished and may 

 even be absent at the fovea. Because of the diminution in the number 

 of ganglion cells in this area the nerve fiber layer is greatly diminished 

 in thickness on approaching the margin of the fovea, and toward its 

 center entirely disappears. 



The fovea centralis lies at the posterior pole of the anteroposterior 

 axis. The light stimulus here meets least obstruction in passing to the 

 neuro-epithelial elements; this is the point of acutest vision. 



Since the fovea centralis contains no rods, it lacks the visual purple; 

 and since vision is sharpest at this point, the visual purple would seem 

 unessential to sight. The 

 visual purple is said to be 

 absent in the eyes of the | 



pigeon, the hen, some rep- Jj ^ 



tiles and some bats. It is 

 supposed to enhance the 

 irritability of the rods in 

 dim lights. 



In the eyes of birds 

 and reptiles the cones out- 

 number the rods; and in 

 certain reptiles, e.g., liz- 

 ards, rods are entirely 

 lacking. In sharks and 

 rays, most nocturnal ani- 

 mals, and the owl, cones 

 are either absent or few in 

 number or rudimentary in 

 structure. According to 

 the view of Kreis (1895) 



A! 



FIG. 560. DEVELOPING ROD AND CONE VISUAL 

 CELLS, FROM THE RETINA OF A 345 MM. (6 MOS.) 

 HUMAN FETUS. 



M, diplosome in a Miiller's fiber at the level of 

 the external limiting membrane; C, diplosome in a 

 cone cell; F, fiber growing out from the cone diplo- 

 some; R, diplosome in a rod visual cell. (Seef elder.) 

 X 1000. 



the cones function in the 



perception of color, the rods are sensitive only to light and darkness. In 



color blindness the cones are defective; in night-blindness the rods are 



affected. 



Development Of Rods and Cones. The rods and cones arise and by 

 an essentially identical mode of differentiation from apparently similar em- 

 bryonic cells. A protoplasmic bud is pushed beyond the external limiting 

 membrane towards the layer of pigmented cells. The cone bud is from 



