CHAP. vi DIOPTEIC MECHANISM OF THE EYE 



ciliary margin of the iris is in relation with the base of Bruch's 

 muscle and with the ciliary processes, and is continuous with 

 the cornea hy the liganientum pectinatum. Between it and the 

 cornea is tlie so-called iridic angle, which varies with the anterior 

 curvature of the iris, and is of great importance; to oculists. The 

 colour of the anterior surface of the 

 iris varies widely between turquoise 

 blue, grey, yellow, and brown. The 

 posterior surface is quite black o wing- 

 to a layer of pigment epithelium, 

 the continuation of that in the ret- 

 The edge of the pupil lies close 



ma. 



to the lens; the ciliary or posterior 



part forms the anterior wall of a 



triangular space filled with aqueous 



humour the posterior chamber. 

 The tissue or stroma of the iris 



consists of cells and fibres of connec- 

 tive tissue, mostly arranged radially 



to the pupil. The specific colour 



which it reflects outwards is due to 



ramified pigmental cells resembling 



those of the choroid. Contiguous to the margin of the pupil there 



is a zone of smooth muscle-fibres circularly disposed, about 0'5 m. 



broad, known as the sphincter pupillae. There is also a layer of 



muscle-fibres, radially disposed and therefore acting antagonistic- 

 ally to the sphincter. They 

 begin at the ciliary or outer 

 edge of tlie iris, at the so- 

 called membrane of Bruch, 

 immediately in front of the 

 pigment epithelium, and 

 converge towards the pupil, 

 where they bend round and 

 lose themselves among those 

 of the sphincter (Fig. 110). 

 The existence of a true dila- 



F]... nn. -Segment of the iris seen from posterior surface tatoi' pupillae has been 

 after removal of tlie uveal pigment. (Ivanon".) a, I i ,1 



of which 'miy the deepesi piiri is questioned by many authors, 

 musele ,,f pupil lying immediately ], ot h j n man an( j mamma l s . 



;. IIIVI. Ciliary processes Seen from 



beliind. Twice the natm.il si/e. 1, 

 posterior surface of iris, witli sphincter 

 ]iui)illae; 2, anterior part of choroul ; 

 3, ciliary processes. 



But it is now generally ad- 

 mitted that even if it does not consist of true muscle cells, like 

 the sphincter, it is a continuous membrane, radially fibrillated, 

 constituted of a specific myoid tissue. 



The rdina is a delicate membrane, of which the posterior part 

 us far as the ora serrata contains tlie nerve-cells, from which the 

 fibres of the optic nerve originate, as well as their end-organs. 



