Page:Structure and functions of the body; a hand-book of anatomy and physiology for nurses and others desiring a practical knowledge of the subject (IA structurefunctio00fiskrich).pdf/75

• 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 surface of the iris is covered with a thick layer of pigment 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 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 constricted 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.

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

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