VISUAL PURPLE. 615 



free as possible from white substance). The white is decidedly richer 

 in neurokeratin than the peripheral nerves or the gray substance. Accord- 

 ing to GRIFFITHS, 1 neurochitin replaces neurokeratin in insects and crus- 

 tacea, the quantity of the first being 10.6-12 p. m. 



The quantity of mineral constituents in the brain amounts to 2.95- 

 7.08 p. m. according to GEOGHEGAN. He found in 1000 parts of the 

 fresh, moist brain 0.43-1.32 Cl, 0.956-2.016 P0 4 , 0.244-0.796 C0 3 , 

 0.102-0.220 S0 4 , 0.01-0.098 Fe 2 (P0 4 )2, 0.005-0.022 Ca, 0.016-0.072 

 Mg, 0.58-1.778 K, and 0.450-1.114 Na. The gray substance yields an 

 alkaline ash, the white an acid ash. MAGNUS-LEVY 2 found in fresh brain 

 substance 1.305 p. m. Cl, 0.166 p. m. Ca, 0.139 p. m. Mg, and 0.083 p.m. 

 Fe. 



Appendix. 



THE TISSUES AND FLUIDS OF THE EYE. 



The retina contains in all 865-899.9 p. m. water, 57.1-84.5 p. m. 

 protein bodies myosin, albumin, and mucin (?), 9.5-28.9 p. m. lecithin, 

 and 8.2-11.2 p. m. salts (HOPPE-SEYLER and CAHN 3 ). The mineral bodies 

 consist of 422 p. m. Na 2 HP0 4 and 352 p. m. NaCl. The retina con- 

 tains, according to BARBiERi, 4 also cholesterin but no cerebrosides and in 

 fact none of the specific constituents of the brain substance. 



Those bodies which form the different segments of the rods and cones 

 have not been closely studied, and the greatest interest is therefore con- 

 nected with the coloring-matters of the retina. 



Visual purple, also called rhodopsin, erythfopsin, or VISUAL RED, is 

 the pigment of the rods. BOLL, S in 1876, observed that the layer of rods 

 in the retina during life had a purplish-red color which was bleached 

 by the action of light. KUHNE 6 later showed that this red color might 

 remain for a long time after the death of the animal if the eye was pro- 

 tected from daylight or investigated by a sodium light. Under Jthese 

 conditions it was also possible to isolate and closely study this substance. 



Visual red (BOLL) or visual purple (KUHNE) has become known mainly by 

 the investigations of KUHNE. The pigment occurs mainly in the rods and only 

 in their outer parts. In animals whose retina has no rods the visual purple is 



1 Compt. Rend., 115. 



2 Geoghegan, Zeitschr. f. physiol Chem.. 1; Magnus-Levy, Bioch. Zeitschr. 24. 



3 Zeitschr. f. physiol. Chem., 5. 



4 Compt. Rend., 154. 



6 Monatsber. d. Kgl. Preuss. Akad., 12. Nov., 1876. 



6 The investigations of Kiihne and his pupils, Ewald and Ayres, on the visual purple 

 will be found in Untersuchungen aus dem physiol. Institut der Universitat Heidel- 

 berg, 1 and 2, and in Zeitschr. f. Biologic, 32. 



