THE SENSE OF SMELL 



539 



exhibits a high specificity in a group which includes 

 amyl and ethyl acetate. Benzene behaves similarly 

 in a group composed of aromatic hydrocarbons. 

 Octane is similarly active in a group of paraffin 

 hydrocarbons and heavy oils. Dipentane, cedarvvood 

 oil and eucalyptus oil (substances belonging to the 

 terpenes and related compounds) likewise give single 

 unit discharges. 



The ability of various substances, including metal 

 surfaces, to adsorb and retain foreign odors has been 

 tested by Deininger & Sullivan (36). The great ma- 

 jority of metal surfaces not only pick up odors, but 

 also modify and distort them, often so severely as to 

 leave little suggestion as to the original contaminant. 

 This perversion of the perceived odor is not related 

 to the purity of the metal in the case of either copper 

 or aluminum. 



Enzyme Theories of Olfaction 



Alexander (11) has suggested that odor-producing 

 substances affect the catalyst balance of the olfactory 

 cells. This theory has been elaborated by Kistiakow- 

 sky (60) in an hypothesis that substances having odor 

 inhibit a reaction requiring a catalyst. These changes 

 in the concentration of reaction products would 

 cause excitation in specific receptor units. Sumner 

 (81) has criticized this concept on the grounds that 

 substances we smell, in the concentrations needed to 

 smell them, would not be likely to have any effect 

 on any known enzyme systems and would require an 

 array of enzymes with new and unusual properties 

 in the olfactory mucosa. Beidler (21) points out that 

 some substances are eff^ective olfactory stimuli in 

 concentrations as low as io~'' molar. 



Even if an odorous substance should inactivate an 

 enzyme, thus causing a change in concentration of 

 certain substances, there is as yet no explanation as 

 to how this change in concentration could stimulate 

 olfactory nerves. Bourne (26) and El-Baradi & Bourne 

 (38, 39} have detected significant amounts of alkaline 

 phosphatase in the olfactory mucosa and in the taste 

 buds of the tongue and have observed that vanillin 

 inhibits this alkaline phosphatase. However, alkaline 

 phosphatase is widely distributed throughout the 

 body. Goldwasser, quoted by Sumner (81), suggests 

 that the energy needed to stimulate olfactory recep- 

 tors may come from Pauling's electrochemical energy 

 source deriving from the modification of bonding 

 angles within a molecule at the time that the molecule 

 goes into solution. 



Radiation Theories of Olfaction 



Theories of electromagnetic radiation or molecular 

 viljration in relation to olfaction have engaged luany 

 workers (20, 37, 74, 84, 90). However, there appears 

 to be little or no experimental foundation for the 

 concept that the essential properties of odor result 

 from radiations inherent in molecular behavior (73). 

 Indeed, a suljstance such as the deuteroxyl counter- 

 part of n-butyl alcohol has exactly the same odor as 

 the original «-lnit\l alcohol, although its infrared 

 adsorption spectrum is different (89). On the other 

 hand, certain (^/- and /-isomers diflfer in smell, although 

 their infrared spectra are identical. No evoked elec- 

 trical responses can ije recorded from the olfactory 

 mucosa if it is covered with a thin plastic membrane 

 which transmits infrared radiation but impedes con- 

 tact between the stimulating particles and the epi- 

 thelium. There is no indication that olfactory re- 

 ceptors can be stimulated unless the odorous material 

 is brought into contact with the epithelium (76). 



Methods oj Odor Measurement 



SUBJECTIVE ME.ASUREMENT TECHNIQUES. SourCCS of 



error in the subjective assessment of odor quality 

 and intensity have been discussed in a historical sur- 

 vey by Wenzel (87). It is obviously difficult to con- 

 trol such factors as the force of the ob.server's inhala- 

 tion in methods invoking the ' sniff technique, nor 

 does the administration of the odorous substance by 

 a stream of air at constant pressure necessarily con- 

 trol mechanical factors in\'olved in the eddying of 

 air towards the olfactory receptors in the upper part 

 of the nasal cavity. 



Only a few of the many subjecti\e methods will be 

 discussed here, since all appear to involve significant 

 possibilities of error in assessment of threshold, and 

 there are conflicting opinions as to their relative 

 merits. A number of early methods, typified by the 

 olfactometer of Zwaardemaker (90), involved sniffing 

 gradually increasing intensities of the odorous sub- 

 stance up to threshold concentration. In attempts to 

 obviate subjective sniffing, both injection of a blast 

 of air and a continuous stream of air have been tested 

 by Elsberg & Levy (40). They defined the absolute 

 olfactory threshold as the minimal blast of odorous 

 air capable of producing a sensation of odor. Although 

 the measurement so obtained is usually expressed in 

 terms of volume, Jerome (55) has suggested that these 

 threshold measurements are dependent on pressure 

 of the air Ijlast rather than on odor intensity. Jones 

 (56) has also found that aerodynamic factors, es- 



