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TEXT-BOOK OF PHYSIOLOGY. 



cells. The anterior surface is covered by a layer of cells continuous 

 with those covering the posterior surface of the cornea. The pos- 

 terior surface is formed by a thin structureless membrane supporting 

 a layer of pigment cells continuous with those lining the chorioid. 

 The color which the iris presents in different individuals depends on 

 the relative amount of pigment in the connective- tissue corpuscles. 

 In blue eyes the pigment is wanting. In gray, brown, and black 

 eyes the pigment is present in progressively increasing amounts. The 



blood - vessels are 

 connected with 

 those of the chori- 

 oid coat. 



The muscle- 

 fibers are of the 

 non-striated variety 

 and arranged in 

 two sets, one cir- 

 cularly, the other 

 radially, disposed. 



The circular 

 fibers are found 

 close to the pupil 

 near the posterior 

 surface of the iris. 

 Contraction of this 

 band of fibers di- 



FIG. 277. CHORIOID COAT OF THE EYE. i. Optic minishes, relaxation 



increases, the size 

 of the pupil. This 

 muscle is known 

 as the sphincter 

 pupillcE or sphincter 

 iridis. 



The radial 

 fibers form a more 

 or less continuous 

 layer in the posterior part of the iris, extending from the margin 

 of the pupil, where they blend with the circular fibers, to the outer 

 border. Contraction of the fibers enlarges the size of the pupil. 

 The muscle is known as the dilatator pupillce. 



nerves exciting the circular fibers to action are the ciliary 

 nerves, axons of nerve-cells located in the dliaryj^pjDhthairm 

 glion. Stimulation of these fibers gives rise to contraction" 

 sphincter and diminution in the size of the pupil. The nerves excit- 

 ing the dilatator fibers are axons of nerve-cells located in the superior 



277. CHORIOID COAT OF THE EYE. i. Optic 

 nerve. 2, 2, 2, 2, 3, 3, 3, 4. Sclerotic coat divided 

 and turned back to show the chorioid. 5, 5, 5, 5. 

 The cornea, divided into four portions and turned 

 back. 6, 6. Canal of Schlemm. 7. External 

 surface of the chorioid, traversed by the ciliary 

 nerves and one of the long ciliary arteries. 8. 

 Central vessel into which open the vasa vorticosa. 

 9, 9, 10, 10. Chorioid zone, n, n. Ciliary 

 nerves. 12. Long ciliary artery. 13, 13, 13, 13. 

 Anterior ciliary arteries. 14. Iris. 15, 15. Vas- 

 cular circle of the iris. 16. Pupil. (Sappey.) 



