DISEASES OF THE EYE. 343 



margin to form the fibrous membrane of tlie eyelids. When complete 

 the orbital cavity has the form of a regular hollow cone, open at 

 its base and closed at the apex. The opening of this cone is directed 

 forward, downward, and outward. Independently of the globe of 

 the eye, this cavity lodges the muscles that move it, the membrana 

 nictitans, and the lacrimal gland. 



The muscles of the eye are seven in number — one retractor, four 

 straight, and two oblique. The retractor oculi envelops the optic 

 nerve between the brain and the ball of the eye and becomes attached 

 upon the external face of the sclerotic tunic. When this muscle con- 

 tracts, it draws the globe back into the orbit, away from the light. 

 The superior, inferior, external, and internal recti or straight muscles 

 are attached to the back part of the orbital sheath and spread for- 

 ward in four bundles over the globe of the eye, where they are in- 

 serted by a fibrous expansion into the sclerotic at the margin of the 

 cornea. When they act singly, they turn the globe either upward, 

 downward, inward, or outward. The great oblique, by its action, 

 pivots the eye inward and upward in the orbit. The small oblique 

 turns the eye outward and downward. 



The eyelids are two movable curtains, superior and inferior, which 

 cover and protect the eye in front. They are attached to the circum- 

 ference of the orbit and have a convex external face formed by the 

 skin and a concave internal face molded on the anterior surface of 

 the eye and are lined by the conjunctiva, which is reflected above and 

 below on the eyeball. The border of each lid is slightly beveled on 

 the inner side and shows the openings of the Meibomian glands. 

 These glands secrete an unctuous fluid, which is thrown out on the 

 border of the lids, the function of which is to facilitate their move- 

 ments and enable them to retain the tears in the ocular cavity. The 

 eyelid is composed of a fibrous inner membrane ending in a stiff arch 

 near the border, a muscle to close the lid, another to open it, the 

 skin externally, and the conjunctival mucous membrane internally. 

 The border of each lid is covered and protected by long hairs to pre- 

 vent floating particles of matter in the atmosphere from gaining 

 entrance to the eye. 



The membrana nictitans, which is also named the third eyelid, 

 winking eyelid, haw, etc., is placed at the inner angle of the eye, 

 whence it extends over the eyeball to relieve it from foreign bodies 

 which may fall upon it. It has for its framework a fibro-cartilage, 

 irregular in shape, thick, nearly prismatic at its base, and thin 

 anteriorly where it is covered by the conjunctiva ; behind, it is loosely 

 attached to a fatty cushion. 



The lacrimal gland is situated between the orbital process and the 

 upper part of the eyeball. It secretes the tears destined to lubricate 



