CH. LV.] OLFACTORY SENSATIONS 787 



to their concentration. Chemical dissociation, too, unquestionably 

 plays a prominent part. 



Nevertheless, there are certain observations which indicate the 

 existence of primary sensations of smell. First, some persons are 

 congenitally insensible to one or more odours, but yet smell others 

 quite normally. Hydrocyanic acid, mignonette, violet, vanilla, 

 benzoin, are substances which appear to certain people to have no 

 smell. Secondly, some odorous bodies, when simultaneously given, 

 antagonise one another; others produce a mixed smell. Thirdly, 

 fatigue of the epithelium with one odour will modify or abolish the 

 effect of some smells, but will leave that of others untouched. 



The delicacy of the sense of smell is most remarkable even in man. 

 Valentin calculates that even 100 Q 000 of a grain of musk can be dis- 

 tinctly smelled. Solutions of camphor afford a good means of testing 

 olfactory acuity. Two tubes of camphor solution are presented to the 

 subject along with two tubes of water, and the former pair is replaced 

 with weaker and weaker solutions until it is indistinguishable from 

 the tubes containing water. Pungent substances, such as ammonia, 

 are unsuited for olfactometrical experiment. They stimulate the 

 endings of the fifth as well as those of the olfactory nerve. 



