PROTECTION OF EYE. 307 



and in the center of this a slight hollow. In this region vision 

 is most distinct, especially for color. 



Protection of the Eye. The outer coat holds all in 

 place, and gives strength for attachment of the muscles that 

 give motion to the eyeball. The Lacrymal Gland, or tear 

 gland, is just above the outer angle of the eye, and pours its 

 secretion over the eyeball in weeping, or when there is need 

 of an unusual supply of tears. The lids serve as curtains to 

 admit or shut out light, and, by winking, with their own secre- 

 tion, a fluid mixture of salt water and mucus, to wash the eye. 

 It is as though a man were kept all the time in front of a 

 plate-glass window, with water and rubber scraper, to keep 

 it clean and bright. The lacrymal secretion is, ordinarily, 

 carried off as fast as it is made, by two ducts beginning at the 

 inner angle of the eye, one on each lid ; these two ducts soon 

 unite, and empty by one outlet into the nasal cavity. If these 

 ducts are stopped, or if the secretion be formed very rapidly, 

 the liquid overflows on the face as tears. Then, too, the eye- 

 ball is set well in its bony socket to shield it from harm. The 

 cornea, or window, is a continuation of the sclerotic coat. 



The choroid coat is richly supplied with blood tubes and 

 dark pigment to absorb light so it may not be reflected about 

 in the eye. The choroid coat, continued forward, becomes the 

 iris, but is incomplete, leaving the circular hole in this dark 

 curtain. The pupil looks dark because we are looking into a 

 dark room. A thin membrane, called the hyaloid membrane, 

 lines the inner surface of the retina. Arising from the scle- 

 rotic coat at the outer border of the iris is a muscle, the ciliary 

 muscle ; it is inserted at the margin of the lens by means of 

 fibrous strands that form an intimate part of the capsule 

 inclosing it. 



Take a single lens that is convex on both sides, such as in - 

 the common tripod lens, or any hand magnifier. Hold this up 



