IfOEM AND PliOTBCTI(_)K OF THE EYEBALL 



45S 



mucous membrane called the conjunctiva,^ which is also 

 folded back to cover the front of the eyeball. The 

 lids are movable, and when lowered form a protective 

 curtain in front of the eyeball. Their edges are lined ^\■ith 

 sebaceous glands, which, by secreting oil, prevent their 

 sticking together when closed. A row of hairs also fringes 

 these lids and serves to keep out dust and furnish a slight 

 shade. These lids are closed by the contractions of a cir- 

 cular muscle (the orbicularis) which acts in part reflexly, 

 as when an object is suddenly brought toward the eye or 

 head. 



As these lids rest against the front of the eyeball, friction 

 would naturally result between the two surfaces when they 

 rulj together. The mucus secreted by the membrane partly 

 prevents this by acting as a lubricant. There is in addi- 

 tion a small gland about the size of an almond (the tear 

 gland) located on the 

 outer and upper side of 

 the orbit, and this gland 

 secretes a salty liquid 

 called the tear fluid or 

 the lachrymal fluid. The 

 gland is also known as 

 the lachrymal gland. 

 This fluid flows over 

 the front of the eyeball 

 assisted in -its progress 

 by the motion of the 

 lids until it reaches 

 the inner angle of the lids next to the nose. Here it is 

 drained off into a small duct (the lachrymal duct) which 



1 Inflammation of this lining causes conjunctivitis, or pink eye. 



Fig. 213 — view of right eye and tear appa- 

 ratus; G, tear gland; C, C, upper and lower 

 tear ducts; Ji, common nasal tear duct. 



