MEMBRANES OF THE EYE. 337 



occupy the external, and the arteries the internal surface of the 

 choroid. From this account, it is obvious that when this coat is 

 boiled in water, the cellular tissue will be converted into glue, 

 while the veins and arteries will remain undissolved. 



5. Within the choroid, and next the vitreous humour, with 

 which it is merely in apposition, lies the retina, which is an ex- 

 pansion of the optic nerve. It is a soft and pulpy membrane. 

 In the living eye it is transparent, but a few hours after death it 

 becomes of a pale-white colour. According to Lassaigne, who 

 analyzed it, its constituents are the same as those of the medul- 

 lary part of the brain. But it contains scarcely iu 5 th of fatty 

 matter, one portion of which contains phosphorus, and cannot 

 be saponified, while the other is capable of being converted into 

 a soap like common fat. The retina contains 



Water, . . 92-9 



Albumen, . 6*25 



Fat, . . . 0-85 



100-0 

 While the optic nerve contains 



Water, . . 70-36 



Albumen, . 22-07 



Cerebrote and fat, . 4-40 



96-83 



6. The pigmentum nigrum is spread upon the choroid, by the 

 inner membrane of which it appears to be secreted. It is easily 

 obtained by washing the choroid coat (freed from the retina) in 

 water as long as that liquid is discoloured. It remains long sus- 

 pended in water, and then appears of a deep-brown colour. 

 But it may be collected on the filter, and then constitutes a black 

 coherent mass. This substance was examined chemically by 

 Berzelius, and some years after by Leopold Gmelin. 



Ber.zelius found it insoluble in water both cold and hot. It 

 was also insoluble in alcohol and in dilute nitric and muriatic 

 acids, as also in concentric acetic acid. Yet these acids assume 

 a shade of yellow. Dilute potash ley dissolves it with diffi- 

 culty, and only after long digestions. The solution is deep-yel- 

 low, and muriatic acid precipitates from it the colouring matter 

 having a light brown colour. 



Y 



