Chap. XX] INTERNAL AND EXTERNAL SENSES 427 



outer circumference of the iris, and is called the dilator of the pupil. 

 The action of these muscle fibres is antagonistic. 



The posterior surface of the iris is covered by a thick layer of 

 pigment-cells designed to darken the curtain and prevent the en- 

 trance of light. The _^ 



anterior surface of the 

 iris is also covered with 

 pigment cells, and it is 

 chiefly these latter 

 which cause the beau- 

 tiful colors seen in the 

 iris. The different col- 

 ors of eyes, however, 

 are mainly due to the 

 amount, and not to the 

 color, of the pigment 

 deposited. 



Function of the iris. 

 — The function of the 

 iris is to regulate the 

 amount of light enter- 

 ing the eye, and thus 

 assist in obtaining clear 

 images. It is enabled 



to perform this function by the action of the muscles described 

 above, as their contraction or relaxation determines the size of the 

 pupil. When the eye is accommodated for a near object, or 

 stimulated by a bright light, the sphincter muscle contracts and 

 diminishes the size of the pupil. When, on the other hand, the 

 eye is accommodated for a distant object, or the light is dim, the 

 dilator muscle contracts, and the pupil is pulled wider open. 



Retina. — The retina, the innermost coat of the eyeball, is 

 the most essential part of the organ of sight, since it is the only 

 one directly sensitive to light. The sclera is the protective, the 

 choroid the vascular, or nutritive, and the retina is the visual, 

 or perceptive, layer of the eyeball. It is a transparent mem- 

 brane of a grayish color that is formed by the spreading out or 

 expansion of the optic nerve. It is situated between the inner 

 surface of the choroid and the outer surface of the vitreous humor, 



Fig. 200. — Choroid Membrane and Iris 

 exposed by the removal of the sclera and 

 Cornea. Twice the natural size, d, one of the 

 segments of the sclera thrown back ; I and k, iris ; 

 c, ciliary nerves ; e, one of the veins of the choroid. 

 The ciliary muscle is crossed by the line from k, and 

 should be represented as radiating. (Collins.) 



