400 NERVOUS SYSTEM. 



cut in half in its long diameter, and is about six lines broad in 

 its middle: the lower one is of a breadth, nearly uniform, of 

 about two lines. Their internal extremities cease just before 

 they reach the puncta lachrymalia, and are attached to the in- 

 ternal palpebral ligament, which has been described as one of 

 the origins of the orbicularis ocuii muscle, at the nasal process 

 of the upper maxillary bone; their external extremities cease 

 just before their commissure, and are firmly attached to the 

 external palpebral ligament. 



These cartilages are thicker where they form the margin of 

 the eyelids, and have there a slope or bevel, by which, when in 

 contact, a small groove is formed on their posterior surface. 

 From their resistance to the concentric contractions of the or- 

 bicularis, they keep the eyelid smooth, and favour its sliding 

 upon the eyeball. Certain animals, being destitute of these car- 

 tilages, when they wink, the skin, by the contraction of the or- 

 bicularis, is drawn up like the mouth of a purse. 



Conjunctiva. Below the pafpebral cartilage is the fourth layer 

 of the eyelid, the conjunctiva. It is a white, thin, and diapha- 

 nous membrane, in the uninflamed state. Beginning at the roots 

 of the cilia, where it is continuous with the skin, it covers the 

 posterior face of each eyelid, is reflected for eight or ten lines 

 towards the bottom of the orbit, and then passes to the eyeball, 

 of which it covers the anterior half, not excepting the cornea. 

 It penetrates into the lachrymal passages, to be continuous with 

 the lining membrane of the lachrymal sac. 



From this description, it is evident that the tunica conjunctiva 

 has one surface presented against itself when the eyelids are 

 closed ; this surface is lubricated and very smooth, so as to per- 

 mit a free motion of the lids and ball of the eye. The other 

 surface is connected in its anterior half by cellular substance to 

 the eyelids, and in its remaining part to the ball of the eye, by 

 the same means. It is united rather loosely to the sclerotica 

 tilr it gets near the margin of the cornea ; but to the latter it ad- 

 heres so firmly, and changes there so much its texture, that it 

 seems like a portion of the cornea. 



This membrane, from its continuity with the skin and the 

 lining membrane of the nose, from its sympathies with thena, 

 from the nature of the discharge from it, and from its extreme 



