STRUCTURE OF THE IRIS 219 



3. Lining the inner surface of the choroid is a very thin trans- 

 parent membrane known as the membrane of B ruck (fig. 250, a}. 



The ciliary muscle of Bowman consists of involuntary muscular 

 bundles which arise at the corneo-sclerotic junction, and pass meri- 

 dionally backwards to be inserted into the choroid (fig. 258, K). Many 

 of the deeper-seated bundles take an oblique direction, and these pass 

 gradually into others which run circularly around the circumference 

 of the iris, and on a level with the ciliary processes. This set of 

 circularly arranged bundles constitutes the circular ciliary muscle of 

 H. Muller (L) ; it is most marked in hypermetropic eyes. 



The iris is that part of the vascular coat of the eye which extends 

 in front of the lens. It is continuous with the choroid and has a 



FIG. 254. SEGMENT OF THE IRIS, SEEN FROM THE POS- 

 TERIOR SURFACE AFTER REMOVAL OF THE UVEAL, 

 PIGMENT. 



, sphincter muscle ; 6, dilatator muscle of the pupil. 



FIG. 255. VESSELS OF THE CHOROID, CILIARY PROCESSES, 

 AND IRIS OF A CHILD. ( 10 diameters.) 



a, capillary network of the posterior part of the choroid, ending 

 at 6, the ora serrata ; c, arteries of the corona ci'iaris, supplying 

 the ciliary processes, d, and passing into the iris, e ; /, the ca- 

 pillary network close to the pupillary margin of the iris. 



similar structure, but its pigment-cells often contain coloured pigment. 

 Besides the homogeneous connective tissue, with numerous elastic fibres 

 and blood-vessels of which it is chiefly composed, it contains two sets 

 of plain muscular fibres. The one set forms the sphincter muscle (fig. 

 245, a], which encircles the pupil, the other set consists of a flattened 

 layer of radiating fibres which extend from the attachment of the iris 

 nearly to the pupil, lying close to the posterior surface and constituting 

 the dilatator muscle (b). 



The back of the iris is covered by a thick layer of pigmeiited 

 epithelium (uvea) continuous with the epithelium of the pars ciliaris 

 retinae. 



The blood-vessels of the iris converge towards the pupil (fig. 255, ej. 



