38 



MANUAL FOR STABLE SERGEANTS. 



The Eye, 



88. The eyes are the organs of sight and are situated in the 

 orbital ca\dties. The eye is spherical in shape and is held in posi- 

 tion by muscles which turn it and retract it (draw it back) in the 

 orbital cavity. A jpad of fat below and behind the eye protects it 



from injury due to 

 blows and causes the 

 memhrana nictitans to 

 pass over the front of 

 the eye when it is re- 

 tracted by the mus- 

 cles. 



89. The eye is pro- 

 tected by two mova- 

 ble fleshy curtains, 

 the upper and lotver 

 eyelids. These are 

 composed of muscular 

 and fibrous tissue in 

 their central portion 

 and are covered ex- 

 ternally with skin. 

 The internal surface 

 of the eyelids, ^ the 

 membrana nictitans 

 and the front part of 

 the eye are covered with a mucous membrane, the conjunctiva, which 

 is continuous with the skin at the edges of the eyelids. In the edges 

 of the eyelids are strong hairs which protect the eye from dust and 

 small objects floating in the air. 



90. The membrana nictitan, or accessory eyelid (haw), is situated 

 between the inner side of the orbital cavity and the eyeball, within 

 the lids. When the eye is retracted it passes over the front part of 

 the eye, removing any foreign objects and moistening thecornea. 



91 . The eye proper is composed of three coats and certain internal 

 structures. 



92. The outer covering of the eye is formed by the sclerotic coat 

 and the cornea. The sclerotic coat is composed of strong white 

 fibrous tissue and forms the protective covering. It covers four- 



FiG. 18. — Section of the eye. 

 i, iris; /, crystalline lens; 

 r, retina. 



c, Cornea; eyelids; /, fluid; 

 0, optic nerve; p, pupil; 



