THE EYE AS AN OPTICAL INSTRUMENT 209 



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 protude (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 strength- 

 ened by plates of fibrocartilage. At the edge of each eyelid the 

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

 ous 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. It has a 

 special muscle to raise it, the levator palpebrce superioris. The 

 eyes are closed by a flat circular muscle, the orbicularis palpebra- 

 rum 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 canthus 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 semilunaris) inter- 

 venes. 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 

 lachrymalis, caused by a collection of sebaceous glands * embedded 

 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 abnormally 

 abundant, and then appears as a yellowish matter along the edges 

 of the eyelids, which often dries in the night and causes the lids 

 to be glued together in the morning. The eyelashes are short 

 * For a description of the glands see p. 479. 



