THE BALL OF THE EYE. 419 



from the vessels. Its particular colour is of a most durable 

 kind. According to the observations of Bichat, the long-con- 

 tinued action of light upon it, when this pigment is transferred 

 from the choroid to a piece of paper, does not affect it ; neither 

 is it changed by being submitted to very strong chemical ageirts, 

 as sulphuric, muriatic, or nitric acid, alcohol, or caustic potash. 

 This degree of indestructibility of colour is an invaluable pro- 

 perty, and almost singular; for it is well known to the keepers 

 of medicinal articles, that the colours of all of them yield to the 

 continued influence of light, and that they also become weaker 

 by the same cause. 



In regard to structure, the choroid coat is thin, soft and easily 

 lacerated : when cleared of its pigment by maceration, it is semi- 

 transparent, and is then seen evidently to consist of but one la- 

 mina; unless we may be disposed to consider as a second one 

 the pigment, naturally on its internal face. It has no appear- 

 ance of fibres in its composition, but, when injected, seems to 

 consist almost wholly of arteries and of veins. 



The arteries are branches of the ophthalmic, and are called 

 ciliary. There are two Long Ciliary Arteries, which pene- 

 trate the sclerotic coat not far from the optic nerve, and pass, 

 one of them, on the external and superior part of the choroides, 

 and the other on its inferior and nasal side, to the front of the 

 eye. In this course, they do not send off any branches of con- 

 sequence till they reach the iris, on which they are distributed. 

 The Short Ciliary Arteries are much more numerous than the 

 others, and also smaller ; their number sometimes amounts to 

 twenty; the most of them penetrate the sclerotica from behind, 

 near the optic nerve also.* They quickly divide into a great 

 number of branches, which depart at very acute angles, and 

 have frequent anastomoses with one another. These branches 

 run forwards, nearly parallel, and, at the fore part of the cho- 

 roides, form a very intricate intertexture, which is continued 

 upon the ciliary processes, and communicates with the vessels 

 of the iris.t 



The veins of the choroid coat are also extremely abundant. 



* Soemmering-, Icones Oculi Human!. t Icones Oculi Humani. 



