U4: 



SPECIAL SENSES. 



FIG. 



The Tears. The secretion of the lachrymal glands is 

 constant, though the quantity of fluid may be increased un- 

 der various conditions. The actual amount of the secretion 



has never been estimated. Dur- 

 ing sleep, it is much diminished ; 

 and, when the eyes are open, the 

 quantity is just sufficient to mois- 

 ten the eyeball, the excess being 

 carried into the nose so gradually 

 that this process is not appreci- 

 ated. That this drainage of the 

 excess of tears takes place is 

 shown by cases of obstruction of 

 the nasal duct, when the liquid con- 

 stantly overflows upon the cheeks, 

 producing considerable inconve- 

 nience. 



The mechanism of the action 

 of the excretory lachrymal appa- 

 ratus is quite simple, though it has 

 been the subject of a good deal of 

 discussion. It is probable that the 

 openings at the puncta lacryma- 



lia 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 back- 

 ward, 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 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 lachrymal secretion : 



Lachrymal canals, lachrymal sac, and 

 nasal canal, opened by their ante- 

 rior portion. 1, walls of the lach- 

 rymal passages, smooth and adhe- 

 rent ; 2, 2, walls of the lachrymal 

 sac. presenting delicate folds of the 

 mucous membrane: 8, a similar 

 fold belonging to the nasal mu- 

 cous membrane. (SAPPET, Traite 

 d'anatomie, Paris, 1871, tome iii., 

 p. 703.) 



