SENSE OF SIGHT. 455 



consists of a limpid, colorless, and alkaline fluid, containing a 

 small quantity of albumen and salts, especially chloride of 

 sodium. The tears are diffused over the eye, from its outer 

 to its inner angle, by means of the movements of the orbic- 

 ular muscle alone ; the winking of the eyes spreads the tears 

 over the conjunctiva; all the surfaces, in fact, which are 

 moistened with the tears, are covered by a mucous surface, 

 called the conjunctiva, which extends from the posterior 

 surface of the eyelids to the anterior surface of the globe of 

 the eye (upper and lower portions of the conjunctiva), and 

 lines the extreme anterior portion of the sclerotic coat, as 

 also the cornea, as we learned while studying that membrane 

 (anterior epithelial coat). The winking of the eyelids thus 

 secures the translucency of the cornea, by the diffusion of a 

 liquid which constantly moistens it, and forms, at the same 

 time, such a delicate and uniform covering that vision is not 

 obscured. Winking may therefore be said to be to the eye 



Fig. 124. Lachrymal apparatus.* 



what deglutition is to the ear (see p. 222), both movements 

 being intermittent and very frequent. One of the earliest 

 effects produced by paralysis of the eyelids is inflammation of 

 the cornea, which, not being protected by the diffusion of the 

 tears, becomes liable to injury from the air and dust. 



* Lachrymal apparatus seen from the conjunctival surface of the eyelids. 

 Meibomian glands are seen to run along the edge of the eyelids. /, Lachrymal 

 gland, c/, Orifices of its seven or eight excretory ducts, at the external angle 

 of the upper conjunctival cul-dc-sac ; at the inner extremity of the edges of the 

 eyelids are seen the orifices' of the lachrymal points (in the lachrymal tubercles). 

 t>, o, Orbicular muscle (orbital portion). 



