THE EYE AS AN OPTICAL INSTRUMENT 249 



cornea, the latter being transparent and situated in front; the 

 former is opaque and white and covers the back and sides of the 

 globe and part of the front, where it is seen between the eyelids 



FIG. 81. The left eyeball in horizontal section from before back. 1, sclerotic; 

 2, junction of sclerotic and cornea; 3, cornea; 4, 5, conjunctiva; 7, ciliary muscle; 

 10, choroid; 11, 13, ciliary processes; 14, iris; 15, retina; 16, optic nerve; 17, artery 

 entering retina in optic nerve; 18, fovea centralis; 19, 20, region where sensory part 

 of retina ends; 22, suspensory ligament; 24, the anterior part of the hyaloid mem- 

 brane; 26, the lens; 29, vitreous humor; 30, aqueous humor. 



as the white of the eye. Both are tough and strong, being com- 

 posed of dense connective tissue. 



The second coat consists of the choroid, 10, the ciliary proc- 

 esses, 11, 13, and the iris, 14. The choroid is made up of blood- 

 vrssels supported by loose connective tissue containing numerous 

 corpuscles, which in its inner layers are richly filled with dark- 

 brown or black pigment granules. Towards the front of the eye- 

 ball, where it begins to diminish in diameter, the choroid is thrown 

 into plaits, the ciliary processes, 11, 13. Beyond these it con- 

 tinues as the iris, which forms the colored part of the eye seen 

 through the cornea; and in the center of the iris is a circular aper- 

 ture, the pupil: so its second coat does not, like the outer one, 

 completely envelop the eyeball. In the iris is a ring of plain mus- 

 cular tissue encircling the aperture of the pupil: when its fibers 

 contract they narrow the pupil. Radiating from this ring to the 



