THE EYELID8. 481 



apparatuses, as, for example, some controlling the light- 

 converging power of the media, and others regulatiflgjLhe 

 size of the aperture (pupil) b^ which jight enters. Out- 

 sl3F!nTl)aTniriim?cle?which bring about its movements, 

 and other parts serving to protect it. 



Each orbit is a pyramidal cavity occupied by connective 

 tissue, muscles, blood-vessels and nerves, and in great part by 

 fat, which forms a soft cushion on which the back of the 

 eyeball lies and rolls during its movements. The contents 

 of the orbit being for the most part incompressible, the eye 

 cannot 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 ves- 

 sels 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 continuous 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 palpe- 

 brce superioris. The eyes are closed by a flat circular mus- 

 cle, the orbicularis palpebrarum, which, lying on and around 

 the lids, immediately beneath the skin, surrounds the aper- 

 ture between them. At their outer and inner angles (can- 

 thi) 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 eye- 



