THE EYE. 399 



Horse. Ox. 



Water 89.99 86.52-87.61 



Solids 10.01 13.43-12.39 



Soluble albumins 4.35 \ g ^_ y Q2 



Insoluble albumins 1.36 j 



Extractives 0.67 0.67- 1.07 



Cholesterin ) 0.65- 0.77 



Lecithin J- 2.39 2.08- 2.89 



Fats ) 0.00- 0.47 



Soluble salts 1.11 0.67- 0.93 



Insoluble salts 0.01 0.02- 0.27 



Like the gray matter of the brain, from which the retina is essen- 

 tially derived, the membrane presents an acid reaction when per- 

 fectly fresh, but becomes alkaline soon after death. 



The albumins which are found in the retina appear to be identical 

 with those of the brain substance, and here, as there, we also meet 

 with neurokeratin. This apparently forms the sheath of the outer 

 portion of the rods. In their interior we meet with protagon, 

 lecith-albumins, and in many animals with a peculiar red pigment, 

 which has been termed rhodopsin. 



Rhodopsin. Of the significance of this pigment nothing is known. 

 It occurs in the outer portion of the rods and is absent in the 

 cones. As the macula lutea, viz., the point of clearest vision, is 

 composed only of cones, we may conclude that its presence is not 

 essential to sight, and, as I have just said, the pigment is not found 

 in all animals. It is absent in chickens, pigeons, in certain reptiles, 

 bats, etc., but is present in owls and deep-sea fishes. On exposure 

 to daylight the pigment fades, and, to isolate the substance, it is 

 necessary to work with sodium light. If the living retina, after 

 having been kept in the dark for some time, is suddenly exposed to 

 an intense light, which is broken in part through the interposition 

 of some dark object, such as the framework of a window, and if the 

 remaining pigment is then fixed with a 4 per cent, solution of alum, 

 red pictures of the interposed object can be obtained on the retina, 

 while the remaining portion has become decolorized. Such pictures 

 are termed optograms. 



The regeneration of the pigment, which is constantly going on in 

 the living animal, is apparently dependent upon the integral union 

 of the layer of rods and cones with the pigmented epithelial layer 

 of the retina ; but of the manner in which this restitution takes 

 place, we know nothing. It appears, however, that its formation is 

 preceded by the development of a yellow pigment, which is termed 

 xanthops'm. 



Of the chemical nature of rhodopsin nothing is known. Be- 

 sides daylight, it is decomposed by acids, alcohol, ether, chloroform, 

 and solutions of the alkaline hydrates, by heating to a temperature 

 of from 52 to 53 C. for several hours, or instantaneously at 76 C. 

 Toward ammonia and a solution of alum it is refractory. 



It is easily soluble in water, containing from 2 to 5 per cent, of 

 Platner's bile. From such a solution it is precipitated on dialysis 



