THE CHOROID. 415 



upon examination, appears to originate from the drying of a 

 fluid effused there. This has been considered as a proof of 

 death ; but there are few practical physicians who have not 

 seen a similar pellicle, during life, in persons who were very 

 weak. 



The Choroid Coat. 



The sclerotica and cornea compose a firm external shell for 

 the eye ; upon which its form depends. 



The sclerotica is lined by a thin, flexible, very vascular mem- 

 brane, denominated Choroides ; which is in contact with it 

 nearly throughout its whole extent. The choroides has been 

 supposed to be derived from the pia mater; but this sentiment 

 is not confirmed by observation ; for the pia mater appears to 

 be connected with the interior surface of the sclerotica coat. 

 The cellular tissue which forms the connection, has been con- 

 sidered by some an extension of the tunica arachnoidea. From 

 its being tinged by the pigment of the choroid, it has been called 

 the membrana fusca. The choroid is so delicate a membrane, 

 and so vascular, that it has been considered by some anatomists 

 as a texture composed entirely of vessels and nerves. It has 

 three sets of arteries which are derived from the ophthalmic 

 branch of the internal carotid, viz. 



1st. The long ciliary arteries, which are generally two in 

 number : they penetrate the sclerotic coat at the posterior part 

 of the eye, and pass, one on each side of the external surface 

 of the choroides, dividing at the ciliary circle, each into two 

 branches, which inosculate with each other around the great 

 circumference of the iris. 



One of the long ciliary arteries (internal inferior) passes 

 on the inner side, just below the horizontal diameter. The 

 other (superior external) passes just above this diameter, on the 

 outer side. It is to avoid this external vessel that, in the usual 

 operation for cataract, the needle is introduced a little below the 

 horizontal level of the eye. The only arteries which are attend- 

 ed by veins, are the two long ciliary. These ciliary veins origi- 

 nate in the iris. The blood which is conveyed by the short 

 ciliary arteries is returned by the vena3 vorticose. 



