OF THE SENSE OF SIGHT. 119 



fibrous tunic of the eyeball (Fig. 67). The globe of the 

 eye rests on a cushion of a fatty cellular substance, and thus 

 these muscles can readily 

 move it in all directions. 

 The nerves, besides the 

 optic, are the third, fourth, 

 a branch of the fifth, and 

 the sixth pairs 



241. Protecting 

 parts of the Eye These 

 are, 1, the osseous orbits. 



243, 2. The eye- 

 brows, the eyelids, and the 

 tears. The eyebrows re- 

 quire no description: their 

 form is sometimes charac- 

 teristic of different races of men. 



243. The eyelids are two in number, and are hori- 

 zontal. A third, when present, is vertical ; they are called 

 upper and lower, and differ in their anatomical structure. 

 Externally they are formed of a delicate integument, sup- 

 ported each by a fibro-cartilage (cartilage tarsi) ; internally 

 they are lined by the membrane called conjunctiva, which, 

 besides forming a layer of the eyelids, invests the front of 

 the eyeball. On the edges of the eyelids are observed 

 cilia or eyelashes, and a little behind these the Meibomian 

 glands are seen ; these secrete a peculiar matter, preventing 

 the adhesion of the eyelids during sleep. Further, in each 

 eyelid the orbicularis muscle or sphincter gives a layer, and 

 in the upper eyelid we find the termination of the muscle 

 called levator palpebrsB superioris, which raises the eyelid 

 from off the surface of the eye. 



The uses of the eyelids are obvious. The sub-mucous cha- 

 racter of the conjunctiva assists in enabling the eyelids and 

 eyeball to play freely on each other; but this is not sufficient 

 and accordingly, to secure to the surface of the eye the requi- 

 site degree of moisture, the tears have been provided. 



244. The lachrymal apparatus is composed of a glan- 

 dular body, placed within the orbit, but exterior to the eyeball. 



* Vertical section of the orbit. a, the cornea; b, the sclerotic; c, the 

 optic nerve ; d, inferior rectus muscle of the eye ; e, rectus superior ; f, rec- 

 tus externus : a portion has been removed to show the optic nerve ; g, ex- 

 tremity of the small oblique muscle ; A, the great oblique or trochleator ; 

 i, levator of the upper eyelid ; k, lachrymal gland. 



