136 LABORATORY MANUAL 



is covered with a firm white coat, the sclerotic, except 

 in front w r here there is a clear layer, the cornea. Look 

 at your neighbor's eye from the side. Does the cornea 

 curve more or less than the remainder of the eye? 

 Directly back of the cornea is the colored part of the 

 eye, called the iris. In the center of the iris is a hole, 

 the pupil. Compare the size of the pupil when in a 

 bright light with its size in a dim light. From this, 

 what do you conclude is the function of the iris? Is 

 its action voluntary or involuntary? 



Now examine the model. Locate the parts mentioned 

 above. The wall of the eyeball consists of three 

 layers : the outer, composed of the sclerotic and the 

 cornea; the middle, the black choroid, and in front 

 the colored iris; and the inner, the retina. Note where 

 the optic nerve enters the two outer coats and spreads 

 out to form the retina. Just back of the pupil note 

 the convex crystalline lens. The space between the 

 lens and the cornea is filled with a liquid, the aqueous 

 humor; and the larger space back of the lens is filled 

 with a jelly-like mass, the vitreous humor. Draw a 

 section of the eye, showing parts given above. 



On the outside of the model locate the six muscles 

 used in moving the eyeball. From the position of each 

 determine what motions it gives to the eyeball. 



B. Hold your pencil point at several different dis- 

 tances from your eye. At what distance can you see 

 it most comfortably and most clearly? From this 

 distance move the pencil toward the eye. At what 



