CHAP, vi DIOPTEIC MECHANISM OF THE EYE 



271 



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 by the ligamentum pectinatum. Between it and the 

 cornea is the 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 owing 

 to a layer of pigment epithelium, 

 the continuation of that in the ret- 

 The edge of the pupil lies close 



FIG. IW. Ciliary processes seen from 

 behind. Twice the natural size. 1, 

 posterior surface of iris, with sphincter 

 papillae ; 2, anterior part of choroid ; 

 3, ciliary processes. 



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 the 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- 

 tator pupillae has been 

 questioned by many authors, 

 both in man and mammals: 

 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 retina is a delicate membrane, of which the posterior part 

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

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



Fio. 110. Segment of the iris seen from posterior surface 

 after removal of the uveal pigment. (Ivanoff.) a, 

 sphincter muscle, of which only the deepest part is 

 seen ; b, dilatator muscle of pupil lying immediately 

 in front of pigment cells. 



