268 HUMAN PHYSIOLOGY CHAP. 



muscles do not balance each other as they should ; the 

 inner may be too strong for the outer and then the eyes 

 turn in toward the nose; this is called " cross-eyed." If 

 the outer muscles are the stronger the person is " wall- 

 eyed." Oculists can cut the muscles just enough to 

 straighten the eyes and make them work together prop- 

 erly. If this is necessary, it should be done in childhood. 



1 . Get a sheep's eye from your butcher and study the eyeball as 

 a whole. Notice the six muscles of the eye. What is the function 

 of each? Notice the cushion of fat around the eyeball. 



2. Remove the muscles and fat and locate the optic nerve, the 

 cornea, and sclerotic coat. 



3. Notice the size and shape of the pupil of the eye. Find the 

 iris and notice its shape and color. 



4. Remove a strip of the sclerotic coat, so that a portion of the 

 choroid may be exposed. 



5. Lift up the choroid with forceps and cut through it carefully. 

 The delicate retina can be seen below. Make drawings in each case. 



185. Appendages of the Eye. The eyeball is set deep 

 in the orbit in order to protect it from injury. Besides 

 this it is provided with eyelids, eyebrows, eyelashes, and 

 tear glands to protect, keep clean, and moisten the more 

 exposed parts of this delicate and important organ. 



The eyelids are made of thin plates of cartilage 

 covered with skin, and lined with a thin mucous mem- 

 brane called the conjunctiva. The conjunctiva also 

 covers the front of the eyeball except the central part of 

 the cornea. The two lids may be brought together and 

 thus cover the eyeball in front, protecting it against 

 foreign bodies. They also distribute moisture over the 

 surface of the cornea. The eyelashes are a row of fine 



