SENSATIONS OF SMELL AND TASTE 561 



smell, as that of ammonia, chlorine, &c., involves stimulation of the 

 nerves of common sensibility, i.e. the fifth nerve, besides stimulation of 

 the olfactory nerve. 



No satisfactory classification of smells has yet been made. The 

 following facts tend to show that there are a number of primitive 

 sensations of smell, as of other sensations : 



(a) Certain individuals, whose olfactory sense is in other respects 

 normal, have no power of distinguishing some odours. 



(6) The olfactory sense is easily fatigued. If it be fatigued so as 

 to be absolutely insensitive for one kind of smell, it is still normally 

 excitable for other smells. 



(c) It is possible by mixing odoriferous substances in certain 

 proportions to annul absolutely their effect on the olfactory organ. 

 Thus 4 grm. of iodoform in 200 grm. 

 of Peruvian balsam is almost odour- 

 less, and the same neutralisation of 

 odours is obtained if the odour of 

 each substance be allowed to act 

 separately on each side by tubes 

 inserted into each nostril. 



For this purpose we may use the in- 

 strument invented by Zwaardemaker, 



called the olfactometer. This consists of FIG. 244. Zwaardemaker's 

 a porous cylinder into which is inserted olfactometer. 



a tube. The porous cylinder is first im- 

 mersed in the fluid whose porous qualities 



are to be tested, and when it is thoroughly soaked it is taken out, dried outside 

 by a cloth, and inside by drawing air through it for a short time. One end 

 of the bent tube is then inserted into the cylinder, which it must accurately 

 fit, while the other end is placed in one nostril. The small wooden screen 

 shown in Fig. 244 serves to shut off the smell of the fluid from the other nostril. 

 When the observer breathes through the bent tube the amount of vapour 

 taken up from the cylinder will depend on the amount of surface exposed, and 

 therefore can be diminished or increased by pushing the bent tube further 

 in, or by drawing it out. If the tube is pushed in so far that the smell is only 

 just perceptible the length of the tube may be measured and taken as the 

 liminal intensity of stimulus for the given substances, in its action on the olfactory 

 nerve-endings. This unit was called by the inventor of the instrument an 

 ' olfactie.' By this means it is" possible to make quantitative estimations 

 of the olfactory sense on one individual and to compare them with observations 

 made on other individuals. By using two such instruments it is possible to 

 present different smells to the two nostrils. One obtains in this way com- 

 bination effects which can be compared to the phenomenon which we shall 

 study later in dealing with binocular contrast. 



36 



