SENSE OF SIGHT. 457 



mal tubes to a siphon. It seems most probable that in the 

 movements of inspiration, the rarefaction of the air in the 

 nasal chambers occasions an aspiration in the nasal duct, and 

 consequently by attraction through the whole series of tubes 

 and sacs ; in the normal state, this slight aspiration is suffi- 

 cient to give rise to the passage of the tears ; thus, when the 

 tears flow in great abundance, we facilitate their passage by 

 short inspirations or sobs. The lachrymal tubes are furnished 

 with valves, varying in number, but all so arranged as to 

 allow the tears to flow in one direction only, and to prevent 

 any reflux. 



Not only does the passage of the air through the nostrils 

 enable us to understand how the tears flow into the nasal 

 tube, but it appears that, on the other hand, the tears serve 

 to lubricate the respiratory organ, and to counteract the dry- 

 ing effect caused by inhalations of dry air; the entrance to 

 the air-tubes is moistened by means of the vapor which is 

 given off in the air inhaled, and the tears apparently assist in 

 maintaining that state of moisture in the lungs which is so 

 favorable to the exchange of the gases (L. Bergeon). The 

 lachrymal system, the product of which always flows into the 

 nostrils, is found even in the ophidia, although their eyeball 

 being hidden behind the tegumentary system, is completely 

 beyond the influence of evaporation. On the other hand, 

 animals, such as the cetaceans, which continually breathe an 

 air saturated with moisture, are the only ones without lachry- 

 mal glands. 1 



1 See A. Estor, " Physiologie Pathologique des Fistules Lacry- 

 males," in " Journ. de 1'Anat. et. de la Physiol.," by Ch. Robin. 

 Janvier, 1866. 



