482 THE HUMAN BODY. 



ball but, near their inner ends, a red vertical fold of con- 

 junctiva, the semilunar fold (plica semilunar is) intervenes. 

 This is a remnant 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. Quite in the inner corner is a reddish ele- 

 vation, the caruncula lachrymalis, caused by a collection of 

 sebaceous glands imbedded in the semilunar fold, Opening 

 along the edge of each eyelid are from twenty to thirty 

 minute compound sebaceous glands, called the Meibomian 

 follicles. Their secretion is sometimes abnormally abun- 

 dant, 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 curved hairs, arranged in one or two- 

 rows along each lid where the skin joins the conjunctiva. 



The Lachrymal Apparatus consists of the tear-gland in 

 each orbit, the ducts which carry its secretion to the upper 

 eyelid, and the canals by which this, unless when excessive, 

 is carried off from the front of the eye without running 

 down over the face. The lachrymal or tear gland, about 

 the size of an almond, lies in the upper and outer part of 

 the orbit, near the front end. It is a compound racemose 

 gland, from which twelve or fourteen ducts run and open 

 in a row at the outer corner of the upper eyelid. The se- 

 cretion there poured out is spread evenly over the exposed 

 part of the eye by the movements of winking, and keeps it 

 moist; finally it is drained off by two lachrymal canals, one 

 of which opens by a small pore (punctum lachrymalis) on 

 each lachrymal papilla. The aperture of the lower canal 

 can be readily seen by examining the corresponding papilla 

 in front of a looking-glass. The canals run inwards and 

 open into the lachrymal sac, which lies just outside the nose, 

 in a hollow where the lachrymal and superior maxillary 

 bones (L and MX, Fig. 26)* meet. From the sac the nasal 

 duct proceeds to open into the nose-chamber below the in- 

 ferior turbinate bone (q, Fig. 89, p. 309). 



Tears are constantly being secreted, but ordinarily in 



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