814 



SPECIAL SENSES. 



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 overhanging 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 covered 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 dis- 

 position of the apparatus just described is shown in Fig. 257. 



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

 fluid may be increased under various conditions. The actual amount of the secretion has 

 never been estimated. During sleep it is much diminished ; 

 and, when the eyes are open, the quantity is just sufficient to 

 moisten the eyeball, the excess being carried into the nose so 

 gradually that this process is not appreciated. That this drain- 

 age of the excess of tears takes place is shown by cases of ob- 

 struction of the nasal duct, when the liquid constantly over- 

 flows upon the cheeks, producing considerable inconvenience. 



The mechanism of the action of the excretory lachrymal 

 apparatus is quite simple, though it has been the subject of a 

 good deal of discussion. It is probable that the openings at 

 the puncta lacrymalia take up the liquid 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 com- 

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

 of Homer, and that this, while it empties the sac, may, in the 

 subsequent relaxation, assist the introduction of liquid from the 

 orbit. 



We know very little with regard to the chemical compo- 

 sition of the tears, beyond the analysis made many years ago 

 by Frerichs. According to this observer, the following is the composition of the lachry- 

 mal secretion : 



FIG. 257. Lachrymal canals, 

 lachrymal sac, and nasal 

 canal, opened by their ante- 

 rior portion. (Sappey.) 



1, walls of the lachrymal pas- 

 sages, smooth and adherent; 

 2, 2, walls Of the lachrymal sac, 

 presenting delicate folds of the 

 mucous membrane; 3, a simi- 

 lar fold belonging to the nasal 

 mucous membrane. 



Composition of the Tears. 



Water 990'60 to 



Epithelium 1-40 " 



Albumen 0'80 " 



Chloride of sodium, 

 Alkaline phosphates, 



Earthy phosphates, \ 7-20 



Mucus, 

 Fat, 



1,000-00 



987*00 

 3-20 

 1-00 



8-80 



1,000-00 



The specific gravity of the tears has never been ascertained. The liquid is perfectly 

 clear, colorless, of a saltish taste and a feebly alkaline reaction. The albumen given in 

 the table is called by some authors, lachrymine, thr&nine, or dacryoline. This substance, 

 whatever it may be called, resembles mucus in many regards and is probably secreted by 

 the conjunctiva and not by the lachrymal glands. It differs from ordinary mucus in being 

 coagulated by water. 



The secretion of tears is readily influenced through the nervous system. Aside from 



