THE SENSE-ORGANS 479 



amphibians to be very far forward, a position retained, with 

 some variation, by Sauropsida. In mammals this becomes 

 greatly modified by the formation of the hard palate, which 

 develops from the fusion of two lateral shelves, beginning 

 anteriorly. This shuts off from the mouth cavity its own 

 primary roof, including the openings of the choanae, and 

 therefore pushes back their communication with the mouth 

 cavity to its posterior limit. A trace of the former communi- 

 cation is retained, however, in many mammals in the form of 

 the naso-palatine canal (Stenson's canal), which opens into 

 the roof of the mouth behind the incisor teeth. A rudiment 

 of this duct occasionally occurs in man, lodged in the incisive 

 canal of the maxillaries. 



For greater efficiency the olfactory surface may be increased 

 in three ways : ( i ) by folding the nasal mucous membrane in 

 an oval or otherwise simple cavity, (2) by complicating the 

 walls of the cavity itself, usually by means of ridges or shelves 

 which may themselves become rolled or variously convoluted, 

 or (3) by the addition of accessory cavities within the ad- 

 jacent bones. The first of these devices is seen in fishes, where 

 the folds are variously disposed, either transverse or longi- 

 tudinal. This folding of the mucous surface may become 

 extremely complex and thus furnish an organ of considerable 

 efficiency. The second and third methods reach their highest 

 development in mammals and are best treated separately. 



The interior of the mammalian nasal cavity, which is usu- 

 ally very large, is by no means a simple space, but is well 

 filled up by projecting folds, composed of thin lamellae of bone 

 covered by mucous membrane. These are termed turtiinalia 

 and come under three categories, in accordance with their 

 relationships to other parts: (i) a naso-turbinal, (2) several 

 ethtno-turbinals, and (3) a maxillo-turbinal. 



The maxillo-turbinal lies ventral and usually anterior to 

 the others, in the pars respiratoria, and in mammals has lost 

 all olfactory function, but is often very complicated and forms 

 a filter or screen to intercept the foreign matter in the air 

 taken in, or to temper it if cold. This is the homologue of the 



