PIGMENTUM NIGRUM. 419 



presents a tesselated appearance. The dark brown pigment 

 wherever deposited in the choroid, the ciliary body or processes, 

 the back surface of the iris, or even in the skin of the black, 

 is found lodged in cells of this description ; and where the 

 coloring is of the deepest hue, the cells will be found in greater 

 numbers, placed in layers one over another. Ruysch gave a 

 description of another membrane, since called the Memhrana 

 Ruyschiana, in which he asserted that he had raised the inner 

 vascular layer at the back part of the choroid where the final 

 ramifications of the vessels take place, as a separate membrane ; 

 anatomists now generally consider this separation to have been 

 artificial. 



— The choroid is still however described as consisting of three 

 layers — an outer one which is principally made up of the veins 

 — a middle one consisting chiefly of minute arteries — and an 

 internal one, the pigmentary lining membrane which includes the 

 pigment in its cells — . 



In the white rabbit, and some other animals, the pigmentum 

 nigrum is entirely deficient ; and the pupil of the eye appears of 

 a red color, owing to the blood-vessels of the choroides.* 



The general connexion between the choroides and sclerotica 

 is very slight ; depending upon a fine cellular substance (the mem- 

 brana fusca before noticed,) and very small blood-vessels and 

 nerves. 



But immediately around the margin of the cornea, the choroides 

 and sclerotica are firmly connected to each other, by the inter- 

 vention of a portion of cellular substance, which although soft, is 

 dense and compact, and of some thickness. As this substance 

 extends round the circumference of the cornea, it necessarily forms 

 a ring ; which is between one and two lines broad. This sub- 

 stance, thus placed, constitutes the Ciliary ligament ; which has, 

 to the great perplexity of students, been called by many different 

 names ; as Orbicularis Ciliaris, Annulus gangliformis, &;c.f 



It is very distinguishable from the choroides, these lerotica, and 



* Its deficiency is also observed in individuals of the human race, called 

 Albinoes or leuccEthiopics. — p. 

 f Discovered by Faiiopius in 1584. 



