. The chordates. Chordata. [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, bu
. The chordates. Chordata. [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 passa
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