IV 



THE SENSE OF SMELL 



1*79 



in the laboratory. They found that 1/23,000,000 mgrm. of this 

 substance diffused in a litre of air gave a feeble but quite distinct 

 olfactory sensation. When we reflect on this extraordinary sensi- 

 tiveness in the rudimentary human olfactory organ, we can obtain 

 some idea of the enormous olfactory acuity of certain animals 

 in which the olfactory mucous membrane is not limited to the nasal 

 cavity but extends as far as the frontal and sphenoid sinuses. 



We owe to Zwaardemaker the invention of a practical method 

 which facilitates quantitative research into the acuity of smell. 

 He gave the name of olfactometry to the investigation of olfactory 

 sensibility for odours in general, and of odorimetry to the measure- 

 ment of the comparative sensitiveness to different specific odours. 



I 



FIG. 68. Indiarubber olfactomefcer connected with a Marey's tambour. (Zwaardemaker.) Con- 

 sists of the olfactometer tube, which runs inside a tube of vulcanised rubber. By pulling 

 this out more or less a different extent of the odoriferous surface is exposed. The tambour 

 records the moment at which inspiration begins. It is connected with a branch of the 

 olfactometer tube by a small receiver which holds pure water, to prevent the smell of the 

 rubber tube of the recording apparatus from affecting the subject. In ordinary examinations 

 of olfactory acuity the recording apparatus is not used. 



As early as 1888 he invented a very simple apparatus, the olfacto- 

 meter, which consists of a graduated glass tube (10 cm. long, 5 mm. 

 wide internal diameter) which runs easily inside a second tube 

 coated on the inner side with some solid odoriferous substance, 

 e.g. vulcanised rubber (Fig. 68). The curved end of the glass tube 

 is introduced into one of the nostrils. If the outer tube is entirely 

 covered by the inner glass tube, no smell is perceived on sniffing 

 through the latter ; but if the glass tube is drawn out so that a 

 greater or less surface covered by the odorous substance is exposed, 

 an odour is perceived on sniffing through the olfactometer; its 

 intensity increases with the area of the surface exposed. 



Zwaardemaker proposes as the unit of qualitative measurement 

 the sensation obtained when the rubber cylinder is exposed for 



