THE FUNCTION OF THE RETINA IN VISION. 855 



of long duration and great intensity, retinal fatigue sets in, beginning 

 earlier in the center than at the periphery. It progresses more quickly 

 at first than later, and is most striking in the morning. 



During direct vision, objects must have an angular velocity of from 

 one to two minutes in a second in order to appear to be in motion. 



The manner in which light acts upon the terminal apparatus of the 

 retina has already been discussed in connection with the visual purple 

 (p. 819). Kiihne showed that by illuminating the retina, actual, 

 permanent pictures could be produced on the retina, for instance the 

 picture of a window, but these gradually disappear. The retina acts 

 in some respects like the sensitive plate of a photographic apparatus, 

 and the light is conceived as having a chemical action, especially as non- 

 illuminated retinas have a more acid reaction than illuminated ones. 



The visual purple is given off by the pigmented epithelium of the retina, as 

 a sort of secretion, to the rods alone, and not to the cones. A bleached retina 

 can take up the purple again if placed in contact with living pigmented epithelium. 

 There are two modifications of the purple in the animal kingdom. The mammalian 

 retina is bleached about sixty times more quickly than the frog's. In fixed 

 rabbits' eyes, under atropin-mydriasis Ewald and Kiihne obtained sharp opto- 

 grams of bright objects at a distance of 24 cm., in from i to i minutes; the 

 picture is fixed by 4 per cent, solution of alum. The visual purple is preserved 

 when dissolved in bile and when saturated with sodium chlorid. It resists all 

 oxidizing agents; zinc chlorid, acetic acid, and mercuric chlorid transform it into 

 a yellow substance. It becomes white only through the action of light; the 

 nonluminous heat rays have no effect; it is decomposed by temperatures above 52. 



Kuhne found that in the illuminated frog's eye the pigment-granules of the 

 pigmented epithelium extend further between the outer segments of ^the rods 

 and cones, and in darkness withdraw again into the outer part of the pigmented 

 epithelial cells. In the eye of the fish exposure to light produces also decrease of 

 the chromatin in the granules and ganglia. 



A further important fact should be mentioned, namely, that the 

 inner segments of the cones become shorter under the influence of light, 

 and elongate in the dark. The action is always bilateral, even when 

 only one eye is exposed to light; but after destruction of the brain, the 

 effect is confined to one side; strychnin-tetanus, thermal, chemical, or 

 electrical irritations act in the same way as light. The optic nerve, there- 

 fore, must contain motor (retinomotor) fibers, in addition to the light- 

 perceiving fibers. Motor phenomena are observed also in the ganglion- 

 cells, and in the outer and inner segments of the rods , the cells of the external 

 nuclear layer changing their form at the same time. In fact these 

 movements cause electrical phenomena in the eye. Isolated inner 

 cone-segments and granules likewise exhibit changes of form when 

 exposed to light. 



According to v. Kries the rods are entirely color-blind, and their 

 chief function is related to vision in weak light ; the cones are for the per- 

 ception of colors. 



In changing from a light to a dark room, or the reverse, the eye 

 must adapt itself first to the action of the light. The eye adapted to 

 light has been found superior in visual activity to the eye adapted to 

 darkness. 



Destruction of the rods or cones of the retina causes corresponding 

 dark spots in the visual field. 



Strong visual impressions render the retina insensitive to light, and 

 permanent injury and blindness may result from necrosis of the ret- 

 inal elements with edema. 



