SENSATIONS OF SMELL AND TASTE 503 



Since the endings of the olfactory cells are bathed in fluid it is eviden 

 that the odorous substances must be dissolved by this fluid before they c- 

 excite the olfactory nerve fibres, and in the case of aquatic animals we know 

 that the projected chemical sense, which we call smell, can only be aroused 

 by substances in solution. It is difficult to show in man that the nerve 

 endings can be excited by solutions. Most of the experiments have been 

 made with solutions which had an injurious effect upon the olfactory 

 epithelium. According to Aronsohn it is possible to excite sensations 

 of smell if the nasal cavity be filled with normal saline fluid, containing 

 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., 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 follow- 

 ing facts tend to show that there are 

 a number of primitive sensations of 

 smell, as of other sensations : FIG. 246. Zwaardemakcr's 



(a) Certain individuals, whose olfac- olfactometer. 



tory 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 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 i 

 inserted into the cylinder, which it must accurately fit, while the other end is plac 

 in one nostril. The small wooden screen shown in Fig. 246 serves to shut off 1 

 of the fluid from the other nostril. When the observer breathes through the I 

 tube the amount of vapour taken up from the cylinder will depend on the amou 



