130 PHYSIOLOGY FOR NURSES 



reflected by or impressed on the retina arc thus in- 

 verted, but our brains have become so accustomed to 

 reverse them that we are never even aware of the fact. 

 It is conceivable that there could be such a disturbance 

 of mental processes as to change this automatic mental 

 action and make us see everything upside down. 



The eyeball consists of an outside, middle, and inner 

 coat. The outer coat is mainly protective, except for 

 its anterior fifth, the cornea, which transmits and re- 

 fracts light. The middle coat is vascular and the inner 

 coat is the active part responding to light as a stimulus. 



The cornea is placed in front of the iris and lens and 

 behind the lens is the vitreous humor which is separated 

 from the retina by a very delicate membrane. The 

 cornea and the lens are the chief refracting media. The 

 iris is the perforated curtain which regulates the 

 amount of light. 



The iris, the colored portion of the eye, has circular 

 muscular fibers which surround the pupil and can, by 

 contracting, decrease its size. There are also radiating 

 fibers which can widen or dilate the pupil. If either 

 should stick to the lens, in front of which they lie, the 

 pupil would be contracted or dilated unequally and 

 appear as a jagged instead of a circular opening. 



The lens, crystalline lens, is surrounded by an elastic 

 membrane called its capsule. This membrane is con- 

 nected with a muscle, the ciliary, which is in turn fas- 

 tened to the middle or choroid coat. In the normal 

 condition, where the eyes are used to view distant ob- 

 jects, the ciliary muscle is at rest and the elastic capsule 

 flattens the lens without fatigue to itself; but when the 

 eye must accommodate itself for near vision, the muscle 

 contracts, draws the choroid forward, relaxing the 

 capsule while the lens, by its own elasticity, becomes 



