THE ORGANS OF SPECIAL SENSE. 



69 



gin contain the ciliary muscles and are known as the 

 ciliary processes, while the name iris is given to the little 

 round pigmented, perforated, curtain-like muscle just 

 in front of the crystalline lens. The posterior sur- 

 face of the iris is covered with a thick layer of pig- 

 ment cells to prevent the entrance of light except 

 through the central opening or pupil, and its anterior 

 surface also has pigment cells that give it its color, 

 though the difference in the color of people's eyes is due 



Ocular muscle. 



Retina. 



Sclera. 

 Choroid.- 



Ciliary muscle. 

 Iris. 



Conjunc. cul-de-sac. 



Ant. chamber and 



aqueous humor. 



Crystalline lens. / 



Posterior chamber. 



Angle of ant. chamber. 



Suspensory ligament 



of the lens. 



Cornea. Vitreous chamber. 



FIG. 25. Vertical section through the eyeball and eyelids. (Pyle.) 



rather to the amount of pigment present than to its 

 color, a small amount of pigment being present in blue 

 eyes and a large amount in brown and black eyes. 

 Variations in the size of the pupil are brought about by 

 contractions of the circular and radiating fibers of the 

 iris, contraction of the circular fibers making it smaller 

 and those of the radiating larger. The pupil is con- 

 stricted for near objects and during sleep, and is dilated 

 for distant objects. In a dull light also it dilates to let 

 in more light, and in a bright light it contracts. The 

 appearance of the pupil is often important as a means 

 of diagnosis and in etherization. 



Lastly there is the innermost sensitive coat or retina, 

 which has eight layers, the outer one containing some 

 pigment cells and the next the rods and cones, in which 



