230 SMELL, TASTE, AND CHEMICAL SENSE IN VERTEBRATES 



fish by acid is the hydrogen ion, the stimulus for the sour taste in man, and in th 

 stimulation of the fish by alkali, the hydroxyl ion, the stimulus for the so-called 



alkaline taste in man. From these various comparisons 



well as from those 



smell 



mentioned in the preceding section, it must be evident that the common chem 

 ical sense and the sense of taste are closely related in vertebrate, in fact they verv 

 probably overlap in a number of respects. 



Although taste and the common chemical sense are undoubtedly closely 

 related physiologically, from this standpoint both are sharply marked off from 



The physiology of the olfactory organs in fishes and other water-inhabit- 

 ing vertebrates has been a matter of much dispute. The idea, for which Weber 

 is chiefly responsible, that solutions of odorous materials cannot be smelled 

 when poured into the nasal chambers, led Nagel (1894) to conclude that the 

 olfactory apparatus in the lower water-inhabiting vertebrates must be stimulated 

 by solutions as the gustatory organ is, rather than as the olfactory organ is, and 

 he, therefore, declared the nasal organs of fishes to be rather organs of taste than 

 of smell. But the work of Aronsohn (1886) and the more recent contributions from 

 Vaschide (1901), Veress (1903) and others, show that Weber 

 probably incorrect and that odorous solutions can be smelled when properly 



view is very 



introduced into the 



Nor is it probable that the olfactory org 



habiting vertebrates are stimulated by any other means than by solutions, for 

 even in such forms the olfactory epithelium is not really exposed to the air, but 

 is covered with a layer of fluid in which any substances carried by the air must 

 first become dissolved before it can reach the submerged olfactory cells. In the 

 fishes and other water-inhabiting vertebrates where the olfactory surfaces are 

 bathed by a current of water, the recent work of Baglioni (1909) and particularly 

 that of my own (Parker, 1910, 1911) and of Sheldon (1911) have shown con- 

 clusively that these organs are stimulated much as they are in the higher verte- 

 brates thus enabling the fishes to scent their food, an act that they could scarcely 

 perform with only an organ of taste. It is, therefore, highly probable that the 

 olfactory organs of all vertebrates, like their other chemical sense organs, 

 organs whose receptors are stimulated by solutions (v. Frey, 1904), but in such 

 a way that they can be used as distance receptors and in strong contrast with 

 the activity of the common chemical sense and particularly with taste in which 

 the stimulus comes from only relatively strong solutions and the response elicited 

 is concerned with more locally restricted actions than those called forth by smell. 



are 



6. Conclusions 



The conclusion of this discussion is to the effect that vertebrates possess at 

 least three classes of chemical receptors, the olfactory organs, the organs of the 

 common chemical sense, and the gustatory organs. The receptor of the olfactory 

 organ is the olfactory cell from the base of which an olfactory fiber passes to its 

 termination in the central nervous organ, the whole constituting an olfactory 

 neurone (Diag. A). The olfactory apparatus is stimulated by extremely dilute 



