THE EYE. 421 



of the lens. The circular opening in the iris is the 

 pupil. Three layers may be recognized in the iris, 

 enumerated from before backward, as follows: i 

 anterior endothelium; 2, stroma with sphincter 

 muscle ; 3, pigment epithelium, or pars iridica retinae. 

 The anterior endothelium is a single layer of cells that 

 is continuous with the posterior endothelium of the 

 cornea. The stroma forms the bulk of the iris and 

 is very vascular and muscular. Large pigment cells 

 are present and a fine reticular tissue. Smooth 

 muscle fibers, the sphincter muscle of the pupil, 

 encircle the pupil. Along the posterior surface 

 radial fibers probably function as a dilator muscle 

 of the pupil. The posterior epithelium, or pars 

 iridica ciliaris, is a direct continuation of the pars 

 ciliaris retinae and extends to the margin of the pupil. 

 It is composed of two layers of cells and both, in this 

 case, are pigmented. 



TUNICA INTERNA. 



The inner tunic is the retina of the eye, which may 

 be divided into ten layers, named from within out- 

 ward as follows : 



1 . Internal limiting membrane. 



2. Layer of nerve fibers. 



3. Ganglion cell layer. 



4. Inner molecular layer. 



5. Inner nuclear layer. 



6. Outer molecular layer. 



7. Outer nuclear layer. 



8. External limiting membrane. 



9. Rods and cones. 

 10. Pigment layer. 



