THE EYE. 
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 (Miiller), was first combated by Morochowetz, 2 
who showed thai chondrin here as elsewhere is a mixture of gelatin and 
a mucinoid material. This bitter substance is named by C. T. Morner, 3 
cornea-mucoid ; its percentage composition is ( ', 50*16; II. 6*97; X. 
1 1! * 7 '. » : S, 2*07; 0, 28*01. It resembles other mucoids very closely in 
its properties (see p. 0.'!). 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 it- 
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. — Cahn 4 gives the following table of the quantitative 
composition of the retina:- of geese: — 
Water . . . . . . . 86 to 89 per cent. 
Solids 14 „ 11 
Proteids (globulin coagulating at 50" C, 
albumin and mucin (?) ) . . . 4 ,, G 
Gelatin . . . . . . 13 , , 17 
Cholesterin 0*3 „ 0*8 
Lecithin 10 „ 2*9 
Fat 0-05 „ 0-5 
Salts 0-7 „ 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 tapt turn lucidum. In some fish this 
contains crystals of guanine; in the ox and sheep it does not."' 
Fuscin is one of the group of black pigments, termed melanins. It 
was investigated by Berzelius, and by Hemtz, 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 Gamgee, "Physiological Chemistry," vol. i. p. 451. 
2 Verhandl. d. naturh.-med. Ver. w Hcidclhcrg, pt. 5, Bd. i. 
3 Ztschr.f. physiol. Ohem., Strassburg. Bd. xviii. S. 213. 
4 Hoppe-Seyler, "Physiol. Chem.," S. 699. 
5 Kiihue and Sewall, Verhandl. d. naturli.-med. Ver. tu Heidelberg, X. 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, Benk-z and Nencki named the pigment phyma- 
torusin ; in those of horses, Mppomelamn. The subject of melanin in the urine has been 
