868 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



existence of pigment cells in the organ of vision. It has been mentioned 

 that the first indication of organs of vision is represented by an accumu- 

 lation of pigment cells, and we never find the presence of an eye which 

 is totally free from such pigment matters. 



A fact which at first seemed to show exactly how this process was 

 accomplished was the discovery in the retina of the purplish-red pigment, 

 the so-called visual purple, or rhodopsin, which is so extremely sensitive 

 to light that by proper means external objects may actually be photo- 

 graphed in it on the retina. This substance may be extracted from the 

 retina by means of a 2.5 per cent, solution of the bile acids, especially if 

 the eyes have been kept for some time in a 10 per cent, solution of com- 

 mon salt. Kiihne stated that by illuminating the retina actual pictures 

 could be produced on the retina, and that they gradually disappeared. 

 The analogy between this fact and the action on the sensitive plate in the 

 photographic apparatus is very striking, and the further behavior of the 

 purple pigment of the retina would perhaps show that it is concerned in the 

 appreciation of light. Thus, if a rabbit is kept in the dark for some time 

 and then killed, and its retina examined by a monochromatic light, it will 

 be found to be of a brilliant purple-red color with the single exception of 

 the macula lutea; on the other hand, exposure to light will result in 

 quickly bleaching it, but it will, however, have its color restored if the 

 eye be again placed in darkness. 



Unfortunately, we are as yet entirely unwarranted in forming any 

 such conclusion or affirming any such close connection between this 

 peculiar substance and the sensation of vision by the fact that the pigment 

 is confined to the outer segments of the rods and is absent in the cones, 

 which we have found to be the most sensitive layer; and is, in fact, absent 

 in the macula lutea, which we have found to be the most sensitive point 

 of the retina. Finally, it is absent in pigeons, hens, and bats, although 

 the retina of the latter only consists of cones, while it is found in both 

 nocturnal and diurnal animals. Finally, it is entirely wanting in animals 

 which undoubtedly see very distinctly, and may be entirely removed 

 from the eyes of certain animals, as the monkey, by prolonged exposure 

 to strong light, when the retina will become completely bleached, but 

 will still apparently be perfectly sensitive to light. We cannot, there- 

 fore, at present explain visual sensations as due to chemical changes 

 occurring in this pigment. The discovery of the retinal pigment is, 

 nevertheless, to be regarded as an advance in the elucidation of this sub- 

 ject, for it is almost impossible to conceive that it is not in some way 

 concerned in vision. 



It has been stated that the two pupils act simultaneously, and 

 as we know the function of the pupil is concerned in regulating the 

 entrance of rays of light to the eye and that vision takes place simul- 



