THE SPECIAL SENSES 



265 



365. The Eyelids and Eyebrows. The eyes are protected 

 and kept clean by their eyelids, eyelashes, and eyebrows. 



The eyelids are thin, flexible covers, or shutters, which 

 protect the front of the eyeballs. They are composed on 

 the outside of skin, which is stiffened at the margin by thin 

 plates of gristle. They are lined on the inner side with 

 a very delicate mucous membrane called the conjunctiva, 



FIG. 164. Muscles of the Eyeball. 



A, attachment of tendon connected with the four recti muscles ; , exfernal 

 rectus, divided and'turned downward, to expose the internal rectus; C, in- 

 ferior rectus ; Z), internal rectus ; , superior rectus ; f, superior oblique ; 

 //, pulley and reflected portion of the superior oblique ; K, inferior oblique ; 

 L and M, portions of the muscle which raises the upper eyelid ; to the right 

 of D and to left on same line are seen cut ends of the optic nerve. 



because it is also joined to the eyeball by a fold. It pours 

 out fluid to prevent friction between the surfaces. This 

 fluid, together with the tears, which are constantly flowing, 

 keeps the cornea moist and free from dust. 



The edges of the eyelids are provided with a fringe of 

 fine hairs, the eyelashes, which shade the eye and shield it 

 from dust. 



