282 PHYSIOLOGICAL CHEMISTRY. 



the action of intense light or more continuous light the picture of 

 bright objects, such as windows and the like, so-called -optograms. 



The physiological importance of visual purple is unknown. It 

 follows that the visual purple is not essential to sight, since it is ab- 

 sent in certain animals and also in the cones. HOLMGREK has 

 further shown that in the eye of a frog or rabbit, whose retina 

 has been previously bleached by continuous light, a now of nega- 

 tive electricity is continually observed in the optic nerve when the 

 eye is exposed to the action of light. 



Visual purple must always be prepared exclusively in a sodium 

 light. It is extracted from the net membrane by means of a 

 watery solution of crystallized bile. The filtered solution is evapo- 

 rated in vacuo or dialyzed until the visual purple is separated. 



The pigments of the cones. In the inner segments of the cones of birds, 

 reptiles, and fishes a small fat-globule of varying color is found. KUHNE has 

 isolated from this fat a green, a yellow, and a red pigment called respectively 

 chlorophan, xanthopJtan, and rhodophan. 



The dark pigment of the epithelium cells of the net membrane, which 

 was formerly called melanin, but since named fuscin by KUHNE, dissolves iu 

 concentrated caustic alkalies or concentrated sulphuric acid on warming, but, 

 like melanine in general (see Chapter XIII), has been little studied. The pig- 

 ment occurring in the pigment-cells of the chorioi'dea seems to be identical 

 with the fuscin of the retina. 



The vitreous humor is often considered as a variety of gelatinous 

 tissue. It contains albumin and, as C. TH. MORNER has shown, a 

 mucoid (see page 234). Among the extractives we find a little urea 

 according to PICARD, 5. p. m., and according to ROHMABW, 0.64 

 p. m. The reaction of the vitreous humor is alkaline, and the 

 quantity of solids amounts to about 11 p. m. The quantity of 

 mineral bodies is about 9 p. m., and the albuminous bodies 0.7 p. m. 

 In regard to the aqueous humor see page 126. 



The crystalline lens. That substance which forms the capsule 

 of the lens has not had its chemical nature closely investigated, 

 but gives a reducible body when boiled with dilute acids 

 (0. TH. MORNER). The real substance of the lens, the lens-fibres, 

 contains as chief constituent a globulin which is nearly related to 

 vitellin arid to which BERZELIUS gave the name crystallin. This 

 globulin may be partially precipitated from a watery extract of the 

 lens by means of C0 2 , and it may be completely separated by satu- 

 rating with MgS0 4 . The statements as to the occurrence of an 

 albumin besides the globulin in the lens are disputed; that at least 



