38 SMELL, TASTE, ALLIED SENSES 



tory current as they are in the sharks and rays, and in 

 the lung-fishes. 



In the air-inhabiting vertebrates each olfactory sac 

 possesses, as in man, an external inlet, the anterior naris, 

 and a posterior outlet, the choana, opening into the mouth 

 or the pharynx. The olfactory sacs are relatively simple 

 in amphibians, but become progressively more compli- 



n 



Fio. 11. Ventral view of the head of a shark (Scy Ilium) showing the olfactory pita in rela- 

 tion to the mouth. 



cated in reptiles and birds, and vastly more so in mam- 

 mals. Here the surface of the sac is enormously extended 

 through the development of lateral folds or conchas which 

 may be further complicated by the production of second- 

 ary folds. In mammals the more ventral of these conchae, 

 those attached to the maxillary bone, are apparently not 

 concerned with olf action, but lie in the purely respiratory 

 region of the nasal chamber. The more dorsal conchae 

 those from the ethmoid bone, serve as olfactory surfaces. 

 It has been shown that in some mammals, as for instance 

 in Orycteropus, there may be upwards of ten olfactory 



