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PHYSIOLOGY. 



coloring matter which is found in the connective tissue of the 

 stroma of the iris, and its varying quantity, give the color to the 

 iris. (3) The posterior limiting layer, or a portion of Bruch's mem- 

 brane. (4) The pigment layer (uvea). This is made up of two 

 layers of cells; the posterior layer is cubical and full of pigment, 

 the anterior layer is flat and contains only a small amount of pig- 

 ment. This pigment-layer is a continuation anteriorly of the pig- 

 ment-layer of the retina. The color of the iris is due to pigmented 

 connective-tissue corpuscles, especially in brunettes. The artery 

 and veins of the iris lie at its periphery. 



OPTIC 



Fig. 330. The Nervous Mechanism of the Iris. 



The pupil is made smaller by contraction of its circular fibers. 

 These belong to the smooth type of muscle-fibers and are innervated 

 by the oculomotor through the medium of its ciliary branches. 



The pupil enlarges through contraction of the radiating fibers 

 of the iris. It is innervated by the ciliary branches derived from 

 the great sympathetic. Sensory nerves are present, coming from 

 the first branch of the fifth, or trigeminus. 



Hence, stimulation of the oculomotor, as well as cutting the 

 sympathetic nerve in the neck, or trigeminus, produces contraction 

 of the pupil. Irritation of the sympathetic causes the pupil to 

 dilate. The normal contraction and dilation of the pupil are reflex 

 movements that are caused by the rays of a very strong or very faint 

 light striking the retina. From the retina the impression is con- 



