STRUCTURE OF THE IRIS. 



267 



of the iris nearly to the pupil, lying close to the posterior surface and 

 constituting the dilatator muscle (b). 1 



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

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

 retinae. 



The blood-vessels of the iris converge towards the pupil (fig. 296, e). 

 Near the pupil the small arteries form a small anastomotic circle, from 

 which capillaries arise and pass still nearer the pupil, around which 

 they form a close capillary network. 





FIG. 298. SECTION THROUGH THE COATS OF THE EYEBALL AT THE POINT OF 



ENTRANCE OF THE OPTIC NERVE. (Toldt.) 



Ve, dural sheath ; Vm, arachnoidal sheath, and Vi, pia-niatral sheath of the optic nerve, 

 with lymphatic spaces between them ; 0, 0, funiculi of the nerve ; L, lamina cribrosa ; 

 A, central artery ; S, sclerotic ; Ch, choroid ; R, retina. The small letters refer to the 

 various parts of the retina, b being the layer of rods and cones, and i that of nerve-fibres. 



A large number of nerve-fibres are distributed to the choroid and 

 iris, probably going chiefly to the muscular tissue (ciliary muscle and 

 sphincter and dilatator iriclis). 



The retina consists of the eight layers shown in the accompanying 

 figure (fig. 297), numbered as they occur from within out. 



The inner surface of the retina, which is smooth, rests upon the 

 hyaloid membrane of the vitreous humour. It is formed of the united 

 bases of the fibres of Miiller, which will be afterwards described. 



The layer of nerve-fibres is formed by the expansion of the optic nerve 

 after it has passed through the coats of the eye (fig. 298). At its en- 



1 The existence of a dilatator is denied by some histologists. 



