CH. LIIL] SMELL 737 



The mucous membrane must be neither too dry nor too moist ; if 

 we have a cold we are unable to smell odours or flavours (which are 

 really odours). When liquids are poured into the nose, their smell is 

 imperceptible, as they damage the olfactory epithelium, owing to the 

 difference of osmotic pressure. But even if a " normal " saline solu- 

 tion of an odorous substance fee substituted, the sense of smell is still 

 lost so long as air-bubbles are carefully excluded from the nasal 

 cavity. It is therefore necessary that odorous substances should be 

 in a gaseous state in order to act upon the olfactory epithelium ; they 

 are propagated mainly by diffusion. 



Generally, the odours of homologous series of compounds increase 

 in intensity with increase of molecular weight, but bodies of very low 

 molecular weight are odourless, while vapours of very high molecular 

 weight, which escape and diffuse slowly have little or no smell. A slight 

 change in chemical constitution may produce marked alteration in 

 the character of the odour of a substance ; certain modes of atomic 

 grouping within the molecule appear to be more odoriferous than 

 others. Attempts have been made to discover the elementary sensa- 

 tions of smell, but hitherto with scant success. Many odours have 

 unquestionably a complex physiological effect. For example, when 

 nitrobenzol is held before the nose, it yields first the smell of helio- 

 trope, next the smell of bitter almonds, and finally the smell of 

 benzene ; just as if different end-organs became successively ex- 

 hausted. Some substances have a very different smell according to 

 their concentration. Chemical dissociation, too, unquestionably plays 

 a prominent part. 



Nevertheless, there are certain points 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 odor- 

 ous 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. Valentin 

 calculates that even 100)0 o Q>000 of a grain of- musk can be distinctly 

 smelled. Solutions of camphor afford a good means of testing 

 olfactory acuity. One tube of camphor solution is presented to the 

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

 with weaker and weaker solutions until it is indistinguishable from 

 the tubes containing water. Pungent substances, like ammonia, are 

 unsuited for olfactometrical experiment. They stimulate the endings 

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



3A 



