584 BRAIN AND NERVES. 



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 CO 3 : 

 0.102-0.220 SO 4 ; 0.01-0.098 Fe 2 (PO 4 ) 2 ; 0.005-0.022 Ca; 0.016-0.072 

 Mg; 0.58-1. 778 K; 0.450-1.1 14 Na. The gray substance yields an alkaline 

 ash, the white an acid ash. 



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 1 ). The mineral bodies 

 consist of 422 p. m. Na 2 HPO 4 and 352 p. m. NaCl. 



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, erythropsin, or VISUAL RED, is 

 the pigment of the rods. BoLL, 2 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 3 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 these 

 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 chiefly occurs in 

 the rods and only in their outer parts. In animals whose retina has no 

 rods the visual purple is absent, and is also necessarily absent in the macula 

 lutea. In a variety of bat (Rhinolophus hipposideros) , in hens, pigeons 

 and newborn rabbits, no visual purple has been found in the rods. 



A solution of visual purple in water which contains 2-5 per cent crys- 

 tallized bile, which is the best solvent for it, is purple-red in color, quite 

 clear, and not fluorescent. On evaporating this solution in vacuo we 

 obtain a residue similar to ammonium carminate which contains violet 

 or black grains. If the above solution is dialyzed with water, the bile 

 diffuses and the visual purple separates as a violet mass. Under all 

 circumstances, even when still in the retina, the visual purple is quickly 

 bleached by direct sunlight, and with diffused light with a rapidity corre- 



1 Zeitschr. f. physiol. Chem., 5. 



2 Monatsschr. d. Kgl. Preuss. Akad., 12. Nov., 1876. 



3 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. 



