182 PHYSIOLOGY CHAP. 



olfactory memory images can be called up is amply proved by the 

 researches of Kiesow on the so-called spontaneous representations. 

 Not all the sensations of which patients, particularly hysterics, 

 complain can be regarded as hallucinatory, merely because they 

 are not perceived by normal individuals. In many cases they 

 depend on a Jiyperosmia or abnormal lowering of the threshold of 

 olfactory sensibility, in consequence of which odours not normally 

 perceptible can be detected. But there was undoubted hallucina- 

 tion in the case of a hysterical woman who was aware of an 

 unpleasant taste of menstrual blood some time before the com- 

 mencement of menstruation. 



The partial temporary anosmia that can be artificially 

 produced if the olfactory apparatus is fatigued by prolonged 

 exposure to different strongly odoriferous substances is of great 

 importance in the classification of odours. 



We know that smell is easily fatigued by long-continued 

 exposure to odorous substances. Anatomists who are in the 

 dissecting room for long periods finally cease to notice the 

 cadaveric odours ; patients with foetid wounds or suppurations 

 cease to smell the foetor that disgusts their nurses ; those who-cure 

 furs or work in drains become accustomed to repugnant smells, 

 and fail to perceive them. 



Aronsohn showed that very strong odours depress the activity 

 of the olfactory apparatus in a few minutes, and that after 

 exhaustion a certain time, at least 1-3 min., is necessary to 

 restore excitability. On sniffing tincture of iodine the smell 

 was appreciated only for 4 minutes, balsam of copaiba for 3-4 

 min., camphor 5-7 min., ammonium sulphate 4-5 min., turpentine 

 5 min. 



Zwaardemaker obtained more exact results with his olfacto- 

 meter. He constructed curves of progressive fatigue of the 

 olfactory organ, when excited by odorous substances of constant 

 intensity over a regularly increasing number of seconds. The 

 measure of fatigue is indicated by the progressive rise of the 

 threshold of excitation, i.e. the minimal stimulus perceptible 

 after repeated stimulations of increasing duration. Fig. 70 shows 

 four curves of olfactory fatigue, two obtained with rubber (at 

 a strength of 10. and 14 olfacties), and two others with benzoin 

 (intensity 3 - 5 and 9 olfacties). The first glance shows that the 

 threshold rises, owing to fatigue of the olfactory sense, with the 

 duration of excitation, and the more rapidly according to the 

 strength of the stimulus. On comparing the two curves obtained 

 with rubber and the two with benzoin, it is seen that the latter 

 causes fatigue far more rapidly than the former, although the 

 intensity of the stimulus was less. 



This olfactory fatigue or exhaustion observed after sniffing 

 odorous substances for a certain time does not extend to all 



