170 OF SIGHT. 



260. The anterior edge of the chorioid is terminated 

 by a cellular belt, called orbiculus ciliaris, by which it 

 adheres firmly to a corresponding groove in the scle- 

 rotic, and from which two other membranes, viz. the 

 iris and ciliary processes, are expanded in a circular 

 form. 



261. The iris, (whose posterior surface is lined by a 

 brown pigment and termed wvea) lies anteriorly to the 

 ciliary processes,, is flat, and washed on all sides by 

 the aqueous humour ; narrower towards the nose, 

 broader towards the temples. Its texture is dense and 

 cellular and contains no vestige of muscular fibre. 

 We must regard it, with Zinn,* as a membrane sui 

 generis, and not as a propagation from the chorioid. 

 The anterior surface is differently coloured in different 

 persons, and, during life, counterfeits a flocculent ap- 

 pearance, f 



262. The blood-vessels of the iris run chiefly on its 

 anterior surface, and are continued in the foetus into 

 the membrana pupillaris,\ which begins to open in the 

 centre at the seventh or eighth month of pregnancy, — 

 when the eyes have acquired some degree of size, and 

 when, probably, the elliptic arches of its vessels begin 

 to be gradually retracted into the inner ring of the iris, 

 which ring I have never been able to perceive distinctly 

 before that period. 



* Comment, Soc. Scient. Gottitig. Tom. far. p. 199. 



t On the remarkable mutual relation of the arteries and nerves of the in- 

 ternal parts of the eye and especially of the rris, see Diet. G. Kieacr, De Ann- 

 ntorpkosi (hull. Gotting. 1804. 4to. 



J This beautiful membrane was first discovered by Francis Sandys — a rcle- 

 hrated maker of anatomical preparations i it was first described and exhibited 

 in a plate by F.vcr. J. Wachendorf. Comnurc. Lifter. JS'or. 1740. Hebd. LA 



