LACHRYMAL APPARATUS. 455 



covered with minute hairs which are sometimes so long as to be distinctly 

 visible to the naked eye. 



Immediately to the outer side of the caruncula is a slight duplicature 

 of the conjunctiva, called plica semilunaris, which contains a minute plate 

 of cartilage, and is the rudiment of the third lid of animals, the membrana 

 nictitans of birds. 



Vessels and Nerves. The palpebrse are supplied internally with arteries 

 from the ophthalmic, and externally from the facial and transverse facial. 

 Their nerves are branches of the fifth and of the facial. 



LACHRYMAL APPARATUS. 



The Lachrymal apparatus consists of the lachrymal gland with its ex- 

 cretory ducts ; the puncta lachrymalia, and lachrymal canals ; the lachry- 

 mal sac and nasal duct. 



The Lachrymal gland is situated at the upper and outer angle of the 

 orbit, and consists of two portions, orbital and palpebral. The orbital 

 portion, about three-quarters of an inch in length, is flattened and oval in 

 shape, and occupies the lachrymal fossa in the orbital plate of the frontal 

 bone. It is in contact superiorly with the periosteum, with which it is 

 closely connected by its upper and convex surface ; by its inferior or con- 

 cave surface it is in relation with the globe of the eye, and the superior 

 and external rectus ; and by its anterior border with the broad tarsal liga- 

 ment. By its posterior border it receives its vessels and nerves. The 

 palpebral portion, smaller than the preceding, is situated in the upper 

 eyelid, extending downwards to the superior margin of the tarsal cartilage. 

 It is continuous with the orbital portion above, and is enclosed in an in- 

 vestment of dense fibrous membrane. The secretion of the lachrymal 

 gland is conveyed away by from eight to twelve small ducts, which run 

 for a short distance beneath the conjunctiva, and open upon its surface 

 by a series of pores about one-twentieth of an inch apart, situated in a 

 curved line a little above the upper border of the tarsal cartilage. 



Lachrymal Canals. The lachrymal canals commence at the minute 

 openings, puncta lachrymalia, seen upon the lachrymal papillaB of the lids 

 at the outer extremity of the lacus lachrymalis, and proceed inwards to the 

 lachrymal sac, where they terminate beneath a valvular semilunar fold of 

 the lining membrane of the sac. The superior duct at first ascends, and 

 then turns suddenly inwards towards the sac, forming an abrupt angle. 

 The inferior duct forms the same kind of angle, by descending at first, and 

 then turning abruptly inwards. They are dense and elastic in structure, 

 and remain constantly open, so that they act like capillary tubes in ab- 

 sorbing the tears from the surface of the eye. The two fasciculi of the 

 tensor tarsi muscle are inserted into these ducts, and serve to draw them 

 inwards. 



The Lachrymal sac is the upper extremity of the nasal duct, and is 

 scarcely more dilated than the rest of the canal. It is lodged in the 

 groove of the lachrymal bone, and is often distinguished, internally, from 

 the nasal duct, by a semilunar or circular valve. The sac consists of 

 mucous membrane, but is covered in and retained in its place by a fibrous 

 expansion, derived from the tendon of the oroicularis, which is inserted 

 into the ridge on the lachrymal bone ; it *s also covered by the tensor tarsi 



