SENSATIONS OF TASTE AND SMELL. 297 



affect the olfactoiy cells must, of course, penetrate into the upper 

 part of the nasal chamber. This end is attained during inspiration, 

 either by simple diffusion or by currents produced by the act of 

 sniffing. It may also happen by way of the posterior nares. In 

 fact, the flavors of many foods, fruits, wine, etc., are olfactory rather 

 than gustatory sensations. Wlien such food is swallowed the poste- 

 rior nares are shut off from the pharynx by the soft palate, but in 

 the expiration succeeding the swallow the odor of the food is con- 

 veyed to the olfactory end-organ. Flavors are perceived, therefore, 

 not during the act of swallowing, but subsequently, and if the nostrils 

 are blocked, as in coryza, foods lose much of their flavor. Simply 

 holding the nose will destroy much of the so-called taste of fruits 

 or the bouquet of wines.* 



Nature of the Olfactory Stimulus. — The fact that smells are 

 transmitted through space like light and sound has suggested the 

 possibility that they may depend upon a vibratory movement of 

 some medium. This view, although occasionally defended in 

 modern times, is apparently entirely incompatible with the facts. 

 The usual view is that odoriferous bodies emit particles which, as 

 a rule at least, are in gaseous form. These particles are con- 

 veyed to the olfactory epithelium by currents in the air or by 

 simple gaseous diffusion, and after solution in the moisture of 

 the membrane act chemically upon the sensitive hairs of the sense 

 cells. All vapors or gases are, however, not capable of acting as 

 stimuli to these cells; so that evidently the odoriferous character 

 depends upon some peculiarity of structure. It is assumed that 

 there are certain groups, "odoriphore groups," which are character- 

 istic of all odoriferous substances and by virtue of which these 

 substances react with the special form of protoplasm found in 

 the hair cells. Haycraftf has formulated certain fundamental 

 conceptions bearing upon the relation between chemical structure 

 and odoriferous stimulation. He has shoAvn that the power to 

 cause smell, like other physical properties, is a periodic function of 

 the atomic weight — that in the periodic system, according to Men- 

 delejeff, the elements in certain groups are characterized by their 

 odoriferous properties; for instance, the second, fourth, and sixth 

 members — sulphur, selenium, and tellurium — of the sixth group. 

 Moreover, in organic compounds belonging to an homologous series 

 the smell gradually changes and, indeed, increases in the higher 

 members of the series, — that is, in those having a more complex 

 molecular structure. 



The Qualities of the Olfactory Sensations. — While we dis- 



* For many interesting; facts concerning smelling and the literature to 

 1895 see Zwaardemaker, "Die Physiologie des Ge»-iichs," Leipzig, 1895. 

 tHaycraft, "Brain," 1888, p. 166. 



