506 THE HUMAN BODY. 



the orbit being for the most part incompressible, the eye can- 

 not be drawn into its socket. It simply rotates there, as 

 the head of the femur does in the acetabulum. When the 

 orbital blood-vessels are gorged, however, the eyeball may 

 protrude (as in strangulation) ; and when these vessels empty 

 it recedes somewhat, as is commonly seen after death. The 

 front of the eye is exposed for the purpose of allowing light 

 to reach it, but can be covered up by the eyelids, which are 

 folds of integument, movable by muscles and strengthened 

 by plates of fibro-cartilage. At the edge of each eyelid the 

 skin which covers its outside is turned in, and becomes con- 

 tinuous with a mucous membrane, the conjunctiva, which 

 lines the inside of each lid, and also covers all the front of 

 the eyeball as a closely adherent layer. 



The upper eyelid is larger and more mobile than the 

 lower, and when the eye is closed covers all its transparent 

 part. Ifc has a special muscle to raise it, the levator palpebrm 

 superioris. The eyes are closed by a flat circular muscle, 

 the orbicularis palpebrarum which, lying on and around the 

 lids, immediately beneath the skin, surrounds the aperture 

 between them. At their outer and inner angles (canthi) the 

 eyelids are united, and the apparent size of the eye depends 

 upon the interval between the canthi, the eyeball itself being 

 nearly of the same size in all persons. Near the inner can- 

 thus the line of the edge of each eyelid changes its direction 

 and becomes more horizontal. At this point is found a small 

 eminence, the lachrymal papilla, on each lid. For most of 

 their extent the inner surfaces of the eyelids are in contact 

 with the outside of the eyeball, but near their inner ends a 

 red vertical fold of conjunctiva, the semilunar fold (plica 

 seniilunaris] intervenes. This is a representative of the third 

 eyelid, or nictitating membrane, found largely developed in 

 many animals, as birds, in which it can be drawn all over the 

 exposed part of the eyeball. At the inner or nasal corner is a 

 reddish elevation, the caruncula laclirymalis, caused by a 

 collection of sebaceous glands imbedded in the semilunar 

 fold. Opening along the edge of each eyelid are from 

 twenty to thirty minute compound sebaceous glands, named 

 the Meibomian follicles. Their secretion is sometimes ab- 

 normally abundant, and then appears as a yellowish matter 

 along the edges of the eyelids, which olten dries in the night 

 and causes the lids to be glued together in the morning. 



\ 



