286 PHYSIOLOGY. 



The Lacrymal Secretion. 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 wink- 

 ing, wash the eye with their own secretion, a fluid mixture 

 of salt water and mucus. 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. 



The External Parts of the Eye. The " white of the 

 eye " is the sclerotic coat. It has blood tubes, but ordina- 

 rily they are not conspicuous. The front part of the eye- 

 ball is covered with the cornea. This is transparent, and 

 the color of the iris shows through the cornea. In the 

 center of the iris is the hole, or pupil, by which Jight enters 

 the interior of the eye. 



The Conjunctiva. The front of the eyeball is covered 

 by a thin, transparent, mucous membrane, the conjunctiva, 

 which turns back and lines the inside of the eyelids. It is 

 highly sensitive. 



The Muscles of the Eyeball. There are six muscles which move 

 the eyeball, four straight muscles (the recti) and two oblique. The 

 four straight muscles arise from the deepest part of the eye socket and 

 pass forward to be attached to the top, bottom, and sides of the ball. 

 Where they are attached, they are flattened out like straps. The in- 

 ferior oblique arises from the inner front part of the orbit and passes 

 outward to attach to the under surface of the eyeball. The superior 



