DISEASES OF THE EYE AND ITS APPENDAGES. 355 



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

 rected forward, downward, and outward. Independently of the 

 globe of the eye, this cavity lodges the muscles that move it, the mem- 

 brana 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 become 

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

 the cornea. When they act singly, they turn the globe either up- 

 ward, 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 fiinction of which is to facilitate their move- 

 ments and enable them to retain the t^ars 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 liorder of each lid is covered and protected by long hairs to pre- 

 vent floating particles of matter in the atmosphere gaining entrance 

 to the eye. 



The memhrana 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, and 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 lubri- 

 cate the anterior surface of the eye. This fluid escapes upon the 

 organ at the outer angle of the lids and is carried between them and 

 the eyeball toward the inner angle. 



