SENSES OF SMELL, TASTE, HUNGER AND THIRST 745 



may be inferred that the odoriferous particles reach the olfactory area 

 by diffusion. Their passage upward, however, may be greatly facili- 

 tated by the act of sniffing which tends to displace the air in the vicinity 

 of the olfactory cells by air drawn upward through the fore part of 

 this cavity. This act is, of course, inspiratory in its character, but it 

 cannot be denied that the aforesaid cells may also be activated by the 

 odoriferous particles derived from food and diverted into the nasal 

 cavity by the expiratory air. The senses of smell and taste frequently 

 act together, supplementing one another. In fact, it frequently hap- 

 pens that we project a sensation to the mouth which has actually 

 arisen in the olfactory cells. The preceding data also serve to explain 

 the long latent period usually intervening between the entrance of the 

 odors into the nostrils and the sensation, the largest part of this period 

 being required for the diffusion of the particles to the olfactory area. 



Regarding the manner in which the olfactory cells are stimulated, 

 little is known. It is evident, however, -that the odorous substances 

 emit particles which in most part are in gaseous form. Having 

 arrived in the vicinity of the olfactory area, they enter into solution 

 with the fluid bathing the lining membrane and eventually with the 

 olfactory cells themselves. But only those bodies are capable of acting 

 upon these cells which contain a chemical binder, the so-called odori- 

 phore group, which possesses a chemical constitution, enabling it to 

 unite with the substance of the olfactory epithelium. 1 Hence, smell is 

 essentially a chemical process, consisting in an interaction between the 

 activating body and the protoplasm of the olfactory cells. It is diffi- 

 cult to show this fact, because it is practically impossible to fill our 

 nasal cavity completely with a fluid which is non-irritating. Aron- 

 sohn, 2 however, claims to have succeeded in evoking sensations of 

 smell by means of isotonic solutions of sodium chlorid to which odorif- 

 erous substances had been added. In support of this chemical 

 theory it might be mentioned that the aquatic animals are in posses- 

 sion of a projected chemical sense of smell which, in the nature of 

 things, can only be evoked by substances held in solution. 



The Power of Reaction of the Olfactory Cells Olfactometry. 

 While the sensations of smell may also be evoked by stimulating the 

 olfactory area with an electrical current, the adequate stimulus is, 

 of course, the odorous molecule. Zwaardemaker 3 has attempted to 

 determine the stimulating quantity of different odoriferous substances 

 by means of an instrument, known as the olfactometer. It consists 

 as a rule of two tubes which are curved at their ends so as to facilitate 

 their introduction into the upper part of the nasal passage. The 

 free ends of these tubes are surrounded by somewhat larger tubes 

 (6 mm. in diameter) which are imbibed with some odorous material. 

 Naturally, the farther the outer tubes are shoved over the inner, the 



1 Haycraft, Brain, 1888, 166; also Pussy, Compt. rendus, 1892. 



2 Archiv fiir Anat. und Physiol, 1886. 



3 Die Physiologie des Geruchs, Leipzig, 1895. 



