SENSATIONS OF TASTE AND SMELL. 299 



tions that seem to be of related quality. It is not impossible, how- 

 ever, that further analysis may succeed in showing that there are 

 certain fundamental qualities in our numerous odor sensations. 

 Our position regarding the odors is similar to that which formerh' 

 prevailed in the case of the taste sensations. It was thought to be 

 impossible to classify these latter satisfactorily on the basis of a few 

 fundamental sensations, but it is now universally accepted that all 

 of our true gustatory sensations show one or more of four primary 

 taste qualities. As was said above, our odor sensations are classi- 

 fied in ordinary life as agreeable or disagreeable, and, indeed, 

 Haller, the great physiologist of the eighteenth century, divided 

 odors along this line into three classes: (1) the agreeable or am- 

 brosial, (2) the disagreeable or fetid, and (3) the mixed odors. In 

 many cases, no doubt, the agreeableness or disagreeableness of an 

 odor depends solely upon the associations connected with it. If 

 the associative memories aroused are unpleasant the odor is dis- 

 agreeable. Thus, the odor of musk, so pleasant to most persons, 

 produces most disagreeable sensations in others, on account of past 

 associations. It is possible, however, that there is some funda- 

 mental difference in physiological reaction between such odors as 

 those of putrefaction and of a violet which may be considered as the 

 cause of the difference in psychical effect. It has been suggested, for 

 instance, that they may affect the circulation in the brain in opposite 

 ways, one producing an increased, the other a decreased flow. 

 This improbable supposition has been shown to be devoid of foun- 

 dation by the observations of Shields.* In his experiments the vascu- 

 lar supply to the skin of the arm was determined by plethysmo- 

 .graphic methods, and it was found that both pleasant (heliotrope 

 perfume) and unpleasant (putrefactive) odors give a similar vascu- 

 lar reaction. Each class, if it acts at all, causes, as a rule, a con- 

 striction of the skin vessels, such as is obtained normally from in- 

 creased mental activity, — a reaction usually interpreted to mean a 

 greater flow of blood to the brain. 



Fatigue of the Olfactory Apparatus. — It is a matter of 

 •common observation that many odors, such as the perfumes of 

 •flowers, quickly cease to give a noticeable sensation when the stimu- 

 lation is continued. This result is usually attributed to fatigue 

 of the sense cells in the end-organ and it is noticeable chiefly with 

 faint odors. One who sits in an ill-ventilated room occupied by 

 many persons may be quite unconscious of the unpleasant odor 

 from the vitiated air, while to a newcomer it is most distinct. 



Threshold Stimulus — Delicacy of the Olfactory Sense. — 

 The extraordinary delicacy of the sense of smell in some of the lower 

 animals is seemingly beyond the power of objective measurement or 

 * Shields, "Journal of Experimental Medicine," 1, 1896. 



