THE EYE. 121 



THE EYE. 



The cornea. A thousand parts of corneal tissue contain 242 of 

 solids, of which 204 consist of collagen, 28 of other organic matters, and 

 10 of ash. 1 



The erroneous idea that the cornea, like cartilage, contains a specific 

 substance called chondrin (Mliller), was first combated by Morochowetz, 2 

 who showed that chondrin here as elsewhere is a mixture of gelatin and 

 a mucinoid material. This latter substance is named by C. T. Morner, 3 

 cornea-mucoid ; its percentage composition is C, 50*16 ; H, 6*97 ; N, 

 1279 ; S, 2'07 ; 0, 28-01. It resembles other mucoids very closely in 

 its properties (see p. 63). The gelatin obtained from the collagen 

 resembles that found elsewhere. The same mucoid and collagen are 

 present in the sclerotic. 



Descemet's membrane is resistant to reagents. Morner terms its 

 chief constituent menibranin. It belongs to the mucoid group. The lens 

 capsule has a similar chemical structure. 



The choroid and iris are principally of chemical interest from con- 

 taining the black pigment which is identical with or nearly related to 

 that in the pigment layer of the retina. 



The retina. Calm 4 gives the following table of the quantitative 

 composition of the retinre of geese : 



Water 86 to 89 per cent. 



Solids 14,, 11 " 



Proteids (globulin coagulating at 50 C., 



albumin and mucin (?) ) . . . 4,6 



Gelatin 13 



Cholesterin 0'3 



Lecithin I'O 



Fat . : . . . . . . 0-05 



Salts . 07 



17 

 0-8 

 2-9 

 0-5 

 1-2 



The pigments of the retina. The black pigment of the retinal 

 epithelium is called fuscin. In some animals the epithelium is free from 

 pigment in part ; this constitutes the tapetum lucidum. In some fish this 

 contains crystals of guanine ; in the ox and sheep it does not. 5 



Fuscin is one of the group of black pigments, termed melanins. It 

 was investigated by Berzelius, and by Heintz, who found it contained a 

 small quantity of iron, by Scherer, who found no iron, and also by Eosow 

 and Sieber. The percentage composition obtained by the various 

 observers shows great discrepancies, and this, taken into account with their 

 methods of preparing the pigment, renders it probable they were dealing 

 with impure substances. The failure to find iron was due to the fact 

 that hydrochloric acid was employed at one stage of the operations, 

 and this dissolves out nine-tenths of the iron. 6 



1 His, quoted by Ganigee, "Physiological Chemistry," vol. i. p. 451. 



2 Verhandl. d. naturh.-med. Ver. zu Heidelberg \ pt. 5, Bd. i. 



3 Ztsclir. f. physiol. Chem., Strassburg, Bd. xviii. S. 213. 



4 Hoppe-Seyler, "Physiol. Chem.," S. 699. 



5 Kiihne and Sewall, Verhandl. d. naturh.-med. Ver. zu Heidelberf}, N. F., Bd. ii. 

 Heft 5. 



6 K. A. H. Morner, Ztschr. f. physiol. Chem., Strassburg, Bd. xi. S. 66. The pigment 

 in the skin of negroes, and in melanotic sarcomata, is closely allied to fuscin. It appears 

 to contain iron. In melanotic sarcomata, Berdez and Nencki named the pigment phyma- 

 torusin ; in those of horses, hippomelanin. The subject of melanin in the urine has been 



