

I 



SENSATIONS OF SMELL AND TASTE 645 



a very small proportion of the odorous substance. To this experiment 

 it has been objected that it is almost impossible to fill the nasal cavities 

 without leaving some air spaces, so that the olfactory sensation obtained 

 might have been due to stimulation of the olfactory cells in such a space. 

 There is however no a priori reason to deny the probability of Aronsohn's 

 conclusions. 



Many olfactory stimuli owe their peculiar character to the simultaneous 

 stimulation of other kinds of nerve endings. Thus a pungent smell, as that 

 of ammonia, chlorine, &c., in- 

 volves 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 primi- 



- 11 r FIG. 320. Zwaardemaker's 



tive sensations ot smell, as ot olfactometer. 



other sensations : 



(a) Certain individuals, whose 

 olfactory sense is in other re- 

 spects normal, have no power of distinguishing some odours. 



(b) 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 their effect on the olfactory organ. Thus 4 grm. of iodoform 

 in 200 grm. of Peruvian balsam is almost odourless, and the same neutralisa- 

 tion 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 instrument invented by Zwaardemaker, called 

 the olfactometer. This consists of a porous cylinder into which is inserted a tube. 

 The porous cylinder is first immersed 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. 320 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 maybe 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 

 combination effects which can be compared to the phenomenon which we have 

 .studied in dealing with binocular contrast. 



stud 



