24 THE IRIS. [BOOK in. 



the ora serrata, and which as we have already said is called the 

 pars ciliaris retinae. 



715. The Iris. Just as the ciliary processes continue forward 

 in a modified form the choroid coat, so the iris continues forward 

 the same coat still further, fresh modifications being introduced ; 

 the iris is like the choroid essentially composed of blood vessels 

 lying in a bed of connective-tissue ; but it has special features 

 of its own. 



The hind surface is covered with a layer of cells loaded in all 

 except albino eyes with black pigment. The amount of pigment 

 is so great that the outlines of the individual cells are greatly 

 obscured and with difficulty distinguished ; but we may probably 

 regard the cells as representing two layers, both loaded with 

 pigment, one the continuation of the pigment epithelium of the 

 retina and the other of the pars ciliaris retinas. That is to say, the 

 cells together correspond to the two layers of the retinal cup, one 

 of which, the outer or posterior, is pigmented over the whole of 

 the extent of the cup, while the other, the inner or anterior, 

 is pigmented only at the back of the iris, and elsewhere forms 

 either the pars ciliaris retinae or the retina itself. Fig. 134 

 The cells cease abruptly at the margin of the pupil, which thus, 

 as we have said, forms the extreme lip of the retinal cup. 



The front surface of the iris is also covered with a layer of 

 epithelium, resting on an inconspicuous basement membrane ; but 

 this epithelium, which is easily detached and so overlooked, con- 

 sists of flat polygonal epithelioid plates, and is really a lymphatic 

 epithelium lining the large lymphatic space called the anterior 

 chamber. 



The body of the iris between the front and hind epithelium 

 differs somewhat in the front and back parts. The front part or 

 anterior layer contains relatively few blood vessels, and consists 

 chiefly of a reticular form of connective-tissue furnished by spindle- 

 shaped branched cells mingled with elastic fibres. The hind part 

 of the body, or, counting the pigment behind as one layer, middle 

 or vascular layer of the whole thickness of the iris, consists largely 

 of blood vessels which are accompanied by imperfect sheaths of 

 ordinary connective tissue, the intervals between the vessels being 

 occupied by a reticular tissue like that of the anterior layer, save 

 that the cells are more abundantly supplied with pigment. Bundles 

 of nerve fibres, also accompanied by connective-tissue, are found in 

 this layer but to a greater extent in the anterior layer. 



Around the margin of the pupil, nearer the hind than the front 

 surface, plain muscular fibres are gathered together in the form of 

 a ring, the sphincter iridis, which compact on its inner edge 

 towards the pupil becomes loose and frayed out on its outer edge, 

 many of the fibres and small bundles curving away from the ring 

 and taking a radial direction. The exact form and relative size of 

 this sphincter muscle differs in different animals. 



