178 PHYSIOLOGY CHAP. 



VIII. Class : Odori repugnanti (Linnaeus) 



(a) Narcotic odours of Suhmaceae, henbane, etc. 



(b) Odour of luigs, ul' ozoena. 



IX. Class: Odori nauseanti (Linnaeus) 



(a) Odour of carrion. 



(b) Faecal odour (scatole). 



VI. Delicacy of smell, or the power of perceiving slight 

 differences in the intensity of odours, is often distinguished from 

 olfactory acuity, or the capacity of distinguishing minimal 

 amounts of odorous substances. But the two expressions may 

 be used indifferently, because acuity practically coincides with 

 delicacy of smell. 



Olfactory acuity differs very much for different odours ; it is 

 measured by determining their liminal values. To find the 

 liminal value Valentin (1855) placed small quantities of odorifer- 

 ous substances in a large glass vessel of known capacity, and 

 approximately determined the minimal quantity required to 

 render the air contained in the flask capable of stimulating the 

 olfactory end -organs; or he mixed the odoriferous fluids with 

 large amounts of water, and then tested by smell the minimal 

 dose of odorous substance that could be appreciated. By these 

 methods he found, e.g., that the minimal perceptible amount of 

 essence of roses is 1/200,000 mgrm., of tincture of musk 

 1/2,000,000 mgrm. 



Fischer and Penzoldt (1887), and Passy (1892), made further 

 experiments on the olfactory acuity to different odours, and 

 perfected the methods employed by Valentin. Passy dissolved 

 the substances in alcohol, and from the stock solutions made 

 very weak dilutions, of which he poured a small drop into an 

 empty litre flask, and then tested by sniffing at the mouth of 

 the flask whether the odour were perceptible. All experimental 

 errors in this research tend to raise the threshold of excitation. 

 The following figures, however, give some idea of the extraordinary 

 delicacy of smell for certain odours : 



mgrm. per litre of air. 



Essence of orange . . 0.00005 0.001 



Essence of wintergreen 

 Rosemary . 

 Ether . " . 

 Camphor 

 Heliotrope . 

 Cuniine 

 Vanilline . 

 Natural musk 



0.000005 0.0004 



0.00005 0.0008 



0.0005 0.004 



5 0.005 



0.1 0.05 



0.05 0.01 



0.05 0.0005 



0.01 0.00005 



Artificial musk (trinitrobutyltoluol) . 0.00001 0.000005 



Fischer and Penzoldt made an interesting experiment on the 

 olfactory acuity of man. They tried to determine the minimal 

 perceptible amount of mercaptan in the air of one of the rooms 



