OLFACTORY ORGANS. 



193 



In air-breathing vertebrates, beginning with the dipnoi, a means is 

 developed for drawing air over the sensory surface, the first traces of 

 which are seen in the elasmobranchs. These frequently have an 

 oro-nasal groove, leading from each naris to the angle of the mouth. 

 In some species this groove is practically converted into a tube by the 

 meeting of the walls below. Beginning with the dipnoi and continuing 

 with the amphibia and amniotes (fig. 194) a similar groove is formed 

 on either side before the formation of skeletal parts. This closes in, 

 the edges of each groove uniting, so that a tube or duct is formed, lead- 

 ing from the naris into the oral cavity, where an internal naris or 

 choana occurs. Later maxillary and premaxillary bones arise ventral 

 to the narial passage, so that the ducts appear to run through the skull. 

 The position of the choanae varies considerably, being just inside the 

 jaws in the amphibia and lower reptiles, farther back in the higher 

 reptiles and the birds and mammals, the nasal passages being cut off 

 from the roof of the primitive mouth by the ingrowth of the palatal 

 processes of the maxillary bones and higher, by similar extensions of 

 the palatines, and in some cases, of the pterygoids (fig. 195). 



Incomplete closure of the oronasal groove results in the deformity known as 

 'hare-lip' externally, while 'cleft palate' is the result of failure of palatines, and 

 sometimes of maxillaries to meet below the nasal passages. 



FIG. 196. FIG 197. 



FIG. 196. Section through the nasal region of Siren, after Seydel. en, nasal cavity, 

 jg\ Jacobson's gland; jo, organ of Jacobson; v, vomer. 



FIG. 197. Section of nose of Cheloni a cauana, af ter Gegenbaur. c, concha; ch, choana; 

 *, inner olfactory groove; n, projection of naris between dotted lines. 



In the dipnoi the olfactory membrane forms a few large folds on 

 the dorsal side of the respiratory duct formed from the oronasal tube. 

 In the amphibia the sensory surface has a similar position on the upper 

 medial surface (fig. 196), with frequently a lateral pocket lined with 

 sensory epithelium, the beginnings of an organ of Jacobson. In the 

 same group glands (inner and outer Jacobson's glands) occur for 

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