THE SKIN. 



171 



5, Looking through the transparent cornea, we see the 

 iris, the colored part. It is very smooth and beautiful. 

 It is made partly of muscle fibers, 



of the kind not subject to the will. 

 Some of them form circles around 

 its center : others run from center 

 to edge, like the spokes of a wheel. 

 When these latter fibers contract, 

 the hole in the center, called the 

 pupil, grows large. When the cir- 

 cular fibers contract, the pupil 

 grows small. The iris is a cur- 

 tain for the eye. Bright light 

 makes it close up the pupil as 

 much as possible. Opium and 

 some other drugs will produce 

 the same effect. Dim light makes 

 it open the pupil. Belladonna 

 produces the same effect. 



The contractions of the iris are 

 good illustrations of reflex actions. 



6, The lids have each a thin 



plate of cartilage in them, to make them firm. The inside 

 of each lid and the surface of the eye are covered with a 

 mucous membrane, called the conjunctiva. The hairs 

 (eyelashes) which grow from their edges help to protect the 

 eye from dust and perspiration. Little sebaceous glands, 

 called Meibomian glands, lie under the lining of the lids, 

 and open on their edges. They oil the eyelashes and 

 the edges of the lids. Under the roof of the outer part 

 of the orbit, resting upon the eye-ball, is the tear-gland 

 (lachrymal gland). About a dozen little ducts from it open 



Fig. 67. 



SECTION OF THE FRONT OF THE 

 EYE. 1. Sclerotic. 2. Cornea. 

 3. Anterior chamber. 4. Pos- 

 terior chamber. 5. Iris. 6. Lens. 



