32 



THE EYE. 



stroma, which in such cases is only slightly coloured or is colourless ; but in the 

 black, brown, and grey eye, the colour is due to the pigment cells scattered through 

 the substance of the stroma itself. 



The muscular tissue of the iris is disposed as a ring (sphincter pupillce) 

 around the pupil, and as rays (dilatator pupillcc) from the sphincter to the 

 circumference. 



^b 



sir. 



str. 



Fig. 39. Two SECTIONS OP THE HUMAN IRIS : A, RADIAL ; B, TAKEN ACROSS THE RADII (G-. Retzius). 



HIGHLY MAGNIFIED. 



6, basement membrane and endothelium of anterior surface; sir., stroma; s. , bundles of fibres 

 of sphincter pupillae, cut across (in A) ; d, layer of dilatator papillae ; r, pigment (pars iridica 

 retinae). 



The sphincter muscle (fig. 35, sp ; 39, s, and fig. 40, a) is a narrow band about 

 0'5 mm. wide, situated close to the pupil posteriorly and consisting of a number 

 of bundles of plain muscular tissue running concentrically with the margin of 

 the pupil. Here the bundles are closely arranged, but further from the margin 

 they are more separated, and form less complete rings. 



The ditator (b\ less apparent than the sphincter, begins at the ciliary or 

 outer margin of the iris, and its fibres form a continuous membrane close to the 

 posterior surface, converging towards the pupil. Here they bend round and blend 

 with the sphincter, some reaching nearly to its inner margin. At their origin 

 at the ciliary margin, they also arch round and take a somewhat circular 

 direction. 



The majority of recent writers on the subject have failed to find evidence of the existence 

 of a distinct layer of dilatator fibres and have consequently denied its existence in man and 



