418 



PIGMENTUM NIGRUM. 



nal surface, and does not come off when it is rubbed gently ; or 

 even when immersed in water ; but it is certain, that a consider- 

 able quantity of it sometimes appears 'on the exterior surface of 

 the choroides, in eyes that are not very recent. 



According to some experiments recently made by M. Mon- 

 dini, and Professor Lavini, the dark-brown color of this pigment, 

 is owing to carbon and oxide of iron ; after incineration, parti- 

 cles of iron were readily attracted from the remains by the 

 magnet.* 



In the eyes of many animals nearly all the ruminantia, the 

 horse, the cetacea, most of the carnivora, and some cartilaginous 

 fishes the choroid presents at its back and lower part, a beauti- 

 ful brilliant membranous appearance, which is called the tapetum 

 lucidum, from its power of reflection. It supplies the place of 

 pigment ; for underneath it the pigment is nearly or quite defi- 

 cient. Its color varies in different animals from greenish blue, 

 to yellow, and silvery white. 



The rays of light in these animals, will therefore make a 

 double impression upon the retina, as they pass through it to 

 the choroid and outwards again by reflection. In man this 

 tapetum does not exist, though the term is sometimes applied 

 to the inner surface of the choroid. According to Dalrymple, 

 the membrane of Jacob, is a double reflected serous membrane, 

 one reflection of which is spread smoothly over the inner sur- 

 face of the choroid in most animals, and seems to confine the 

 pigment and prevent its staining the retina, and which, in 

 maceration, breaks up and carries off the pigment in shreds. 

 This appears to be the same membrane described by Mondini, 

 (Membrana Mondini,} as a magnificent and delicate net-work, 

 with meshes so fine as to prevent the passage of a single 

 globule of the pigment to sully in the least degree the delicate 

 retina. It is now however well known that this inner surface 

 of the choroid, is formed of a series of cells which enclose the 

 pigmentary matter. These cells are hexagonal in shape and 

 form a continuous membrane which under the microscope 



* Anatomy of the Eye by J. Dalrymple, London, 1836. 



