452 RESPIRATION. 



the basis of the external wall of the orifice, technically called 

 the alae nasi. The false nostril, a peculiarity of solipedes, is 

 a cavity within and on the supero-external aspect of each 

 orifice ; it is almost completely separated from the corres- 

 ponding nasal chamber, and lined throughout by fine skin, 

 similar to that surrounding the opening. The posterior ori- 

 fices of the nostrils are generally open, though when a pellet 

 of food is passing they are closed by the soft palate, a mus- 

 culo-membranous fold, projecting downwards and backwards 

 from the posterior border of the floor of the nasal chambers. 

 This structure attains its greatest length in solipedes, in 

 which it entirely prevents the process of breathing through 

 the mouth; in these, as in other animals, however, it allows 

 the ready passage of agents from the mouth into the pharynx, 

 and in this case is pushed upwards against the posterior nares, 

 effectually preventing the introduction of foreign bodies into 

 the nostrils. The nasal chambers themselves are two consi- 

 derable cavities, separated from each other by a median osseo- 

 cartilaginous plate, composed of an elongated and slightly 

 flattened bone (the vomer) inferiorly, and of a cartilaginous 



Fig. 125. Vomer. 



septum, continuous with the superior border of the latter. 

 The roof, floor, and external wall of each chamber are com- 

 posed of various facial bones, the frontal, superior, and 

 anterior maxillary, nasal, and palatine. Each nostril is im- 

 perfectly separated into three subdivisions or meati, a supe- 

 rior, median, and inferior, by the two turbinated bones. 

 These are each formed of an extremely thin and fragile per- 

 forated lamina, rolled, as it were, upon itself, and attached 



