OF THE SENSE OF SIGHT. 



125 



k 



Fig. 81.* 



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, 

 requiring no special de- 

 scription here. 



242. 2. The eye- 

 brows, the eyelids, and 

 the tears. The eyebrows 

 require no description : 

 their form is sometimes 

 characteristic of different 

 races of men. 



243. The eyelids 

 are two in number, and 

 are horizontal. 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 fibre-cartilage (cartilago 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 palpebrse superioris, which raises the eyelid from off 

 the surface of the eye. 



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

 character 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. 

 From this so called lachrymal gland, a few ducts proceed 

 and penetrate through the conjunctiva towards the outer 



* 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 y rec- 

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

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

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



