OLFACTORY ORGAN. 



173 



receives a special branch of the olfactory nerve, so that in these 

 cases two separate nasal cavities may be distinguished. This will 

 be referred to again later on (p. 178). 



A farther neomorph are the internal nostrils (choanae) already 

 mentioned, as well as the glands lying under the olfactory 

 mucous membrane ; these are either diffused, or united to form 



FIG. 143. OLFACTORY ORGAN OF Menobranclius laMralis. (From the dorsal side. ) 



JV, olfactory sac ; 01, olfactory nerve ; Pmz, premaxilla ; F, frontal ; P, process of 

 the parietal ; PP, palato-pterygoid ; AF, antorbital process. 



FIG. 144. TRANSVERSE SECTION THROUGH THE OLFACTORY CAVITIES OF 



Plethodon glutinosus (Myctodera). 



S S olfactory mucous membrane ; N, main nasal cavity ; K, maxillary cavity ; C, 

 cartilaginous, and S\ fibrous portion of the turbinal, which causes the olfactory 

 epithelium (E) to project far into the nasal cavity ; ID, intermaxillary gland, 

 shut off from the cavity of the mouth by the oral mucous" membrane (MS) : 

 F, frontal ; Pf, prefrontal ; M, maxilla ; Fop, vomero-palatine ; Sp, nasal 



definite organs. They either open directly into the nasal cavity, 

 their secretion serving for the necessary moistening of the mucous 

 membrane (which is effected in Fishes and Dipnoi by the exter- 

 nal medium), or they pour their secretion into the pharynx or 

 posterior nostrils. 



The latter always lie tolerably far forwards on the palate, and 

 are for the most part enclosed by the vomer, and sometimes by the 

 palatine also. 



