[98 



Basic Structure of Vertebrates 



mouth and pharynx, and in some bony fishes occur even in the external 

 skin of the trunk and fins. These external taste-buds, however pos- 

 terior they may be, are innervated by cutaneous branches of the facial 



nerve (VII). In terrestrial vertebrates 

 the taste-buds are more or less re- 

 stricted to the tongue and roof of 

 mouth. 



Olfactory Organs. There are al- 

 ways separate right and left nasal 

 cavities except in adult round-mouthed 

 eels (cyclostomes), in which the two 

 embryonic nasal sacs become joined 

 into a single median chamber, but with 

 two olfactory nerves. 



The epithelial lining of the olfactory 

 area of the nasal surfaces consists of a 

 single layer of tall columnar cells 

 among which are dispersed the olfac- 

 tory sense-cells, which are of ex- 

 tremely attenuated form (Figs. 169, 

 170), each usually having at its free 

 end one or more fine "hairs" (Fig. 167). 

 The deep end of each olfactory cell 

 continues into the conductor-fiber. The numerous olfactory fibers from 

 one nasal chamber constitute collectively the olfactory nerve, which 

 passes back into the olfactory lobe of the telencephalon, where 

 the fibers make synaptic connections with neurons passing into more 

 posterior parts of the brain. 



The olfactory cells are the only vertebrate sense-cells which 

 produce conductors. Among invertebrates, however, cells of this type 

 are common. It would seem, therefore, that the vertebrate olfactory 

 organ is more primitive in its nervous arrangement than the other 

 sense-organs. 



The nasal chambers in the great majority of fishes do not com- 

 municate with the mouth or other cavities. There are various devices, 

 valvular and otherwise, which serve to introduce external water into 

 them or maintain a current of water in and out. In the small group of 

 lungfishes (Dipnoi) and in all vertebrates other than fishes, each nasal 

 chamber opens into the mouth or pharynx dorsally by an internal 

 naris or choana. The nasal passage thus acquires a respiratory func- 

 tion, air entering at the external naris (nostril) and passing back into 

 the lungs via the trachea. The choanae pierce the anterior region of 

 the roof of the mouth, except in crocodilians and mammals. In these 



Fig. 170. Isolated cells of the 

 olfactory mucosa of a rabbit. 

 (b) Cells of olfactory glands; 

 (/) ciliated cell; (r) olfactory 

 cells; (/•') olfactory cell from 

 which the lower process has been 

 torn off; (s) extruded mucus re- 

 sembling cilia; (st) sustentacular 

 cells. ( X 560.) (Courtesy, Bremer: 

 "Text-Book of Histology," Phila- 

 delphia, The Blakiston Com- 

 pany.) 



