THE LACHRYMAL APPARATUS 



727 



It receives nervous filaments from the fifth cranial nerve and the sympa- 

 thetic. 



The channels by which the excess of tears is conducted into the nose be- 

 gin by two little points, situated on the margin of the upper and the lower 

 lid, near the inner canthus ? called the puncta lacrymalia, which present each 

 a minute orifice. These orifices open respectively 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 mm.) 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 

 apparatus is fibrous and is lined by a reddish, mucous membrane, which 

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

 for each lachrymal canal. Another pair of folds exists near the horizontal 

 portions of the canals. At the opening of the duct into the nose, is an over- 

 hanging fold of the nasal, mucous membrane. These folds are supposed to 

 prevent the reflux of fluid from the lachrymal canals and the entrance of 

 air from the nose. The mucous membrane of the lachrymal canals is cov- 

 ered by a flattened epithelium, like that of the conjunctiva. The lachrymal 

 sac and duct are lined by a continuation of the ciliated epithelium of the 

 nose. The disposition of the apparatus just described is shown in Fig. 

 262. 



The Tears. The secretion of the lachrymal glands is constant, although 

 the quantity of fluid may be increased under various conditions. The actual 

 quantity of the secretion has never been estimated. During sleep it is much 

 diminished ; and when the eyes are open, the quantity 

 is sufficient to moisten 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, producing considerable inconven- 

 ience. 



It is probable that the openings at the puncta 

 lacrymalia take up the lachrymal fluid, like delicate 

 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 mus- 

 cle of Horner, and that this, while it empties the sac, 

 may in the subsequent relaxation assist the intro- 

 duction of liquid from the orbit. 



Very little is known with regard to the chemical composition of tears, be- 



FIG. 262. Lachrymal canals, 

 lachrymal sac and nasal 

 canal, opened by their ante- 

 rior portion (Sappey). 



1, walls of the lachrymal pas- 

 sages, smooth and adher- 

 ent ; 2, 2, walls of the lach- 

 rymal sac, presenting deli- 

 cate folds of the mucous 

 membrane; 3, a similar fold 

 belonging to the nasal mu- 

 cous membrane. 



