THE ORGANS OF SPECIAL SENSE. 



67 



longitudinal folds of skin, the one above, the other be- 

 low, which close like curtains over the eye. Beneath 

 the external layer of skin in the lids is fatty tissue and 

 then the orbicularis palpebrarum muscle by means of 

 which they are closed. They are kept in shape by the 

 tarsal plates or cartilages, in whose ocular surface are 

 embedded the Meibomian glands, whose secretion pre- 

 vents the free edges of the lids from sticking together. 

 Along these edges grows a double or triple row of stiff 

 hairs, the eye-lashes, which curve outward so as not to 

 interfere with each other and also to prevent the en- 

 trance into the eye of foreign bodies. Lining the 

 inner surface of the lids and reflected thence over 



Levator of the upper eyelid 



Superior rectus. 

 External rectus. 

 Inferior rectus. 



Superior oblique. 

 Internal rectus. 



Inferior oblique. 



FIG. 23. The external ocular muscles. (Pyle.) 



the anterior surface of the sclerotic coat of the eye 

 is a mucous membrane, the conjunctiva, which is thick, 

 opaque, and vascular on the lids but thin and transpar- 

 ent on the eye-ball. The angles between the lids are 

 known as the internal a-nd the external canthus. 



Muscles and Nerves. The eyeball is held in posi- 

 tion by the ocular muscles, the conjunctiva, and the 

 lids, while surrounding it, yet allowing free movement, 

 is a thin membranous sac, the tunica vaginalis oculi. 

 The superior and inferior recti muscles at the upper 

 and lower edges of the ball turn the eye up and down; 

 the internal and external recti at the inner and outer 



