THE LACHRYMAL APPARATUS 



709 



malia, which present each a minute opening. These open respec- 

 tively into the upper and the lower lachrymal canals, which together 

 surround the caruncula lacrymalis. At the inner angle just beyond the 

 caruncula, the two canals join to empty into the lachrymal sac, which 

 is the dilated upper extremity of the nasal duct. The duct is about 

 half an inch (12.7 millimeters) in length and empties into the inferior 

 meatus of the nose, taking a direction nearly vertical and inclined 

 slightly outward and backward. This portion of the lachrymal appara- 

 tus is fibrous and is lined by a reddish mucous membrane that presents 

 several well-marked folds. Near the puncta are two folds, one for 

 each lachrymal canal. Another pair of folds ex- 

 ists near the horizontal portions of the canals. 

 At the opening of the duct into the nose, is an 

 overhanging fold of the nasal mucous membrane. 

 These folds are supposed to prevent the reflux of 

 liquid from the lachrymal canals and the entrance 

 of air from the nose. The mucous membrane of 

 the lachrymal canals is covered with flattened 

 epithelium like that of the conjunctiva. The 

 lachrymal sac and duct are lined with a continua- 

 tion of the ciliated epithelium of the nose. The 

 disposition of the apparatus just described is shown 



jj-j "pio- 182 

 J> ' ' 



Ihe lears. The secretion of the lachrymal 

 gland is constant, although its flow may be 

 increased under various conditions. The actual 



i:: 2 



pig. 182. Lachrymal 



canals, lachrymal sac and 

 nasal canal, opened by their 



anterior portion 



i, wails of the lachrymal 



quantity of the secretion has never been estimated, rymai sac, presenting delicate 



-,->. 1 ... i T i i 11 i folds of the mucous mem- 



During sleep it is much diminished ; and when the brane; 3i a similar fold be- 

 eyes are open, the quantity is sufficient to moisten lon g in g to the nasal mucous 



. , membrane. 



the eyeball, the excess being carried into the nose 



so gradually that this process is not appreciated. That this drainage of 

 the excess of tears takes place, is shown by cases of obstruction of the 

 nasal duct, when the liquid constantly overflows upon the cheeks, pro- 

 ducing considerable inconvenience. 



It is probable that the openings at the puncta lacrymalia take up 

 the lachrymal secretion, like little pipettes; this action being aided by the 

 movements in winking, by which, when the lids are closed, the points 

 are compressed and turned backward, opening and drawing in the tears 

 when the lids are opened. It is possible that the lachrymal sac is 

 compressed in the act of winking, by the contractions of the muscle of 

 Horner; and that this, while it empties the sac, may in the subsequent 

 relaxation assist in the introduction of liquid from the orbit. 



