252 SMELL. [BOOK in. 



meter for measuring olfactory sensations has been constructed, 

 the measurements In-in^ L!,iven by the size of the superficial area, 

 impregnated with an odoriferous substance, over which the air 

 must pass in order to give rise to a distinct sensation. The limit 

 of increase of sensation however is soon reached, a minute quantity 

 producing the maximum of sensation and further increase giving 

 rise to exhaustion. The minimum quantity of material required 

 to produce an olfactory sensation may be in some cases, as in 

 that of musk, almost immeasurably small. 



In ordinary circumstances odoriferous particles reach both 

 nostrils, and we receive two sets of olfactory nervous impulses, 

 one along each olfactory bulb. These however are fused into one 

 sensation ; our olfactory sensations are almost exclusively binasal. 

 When two different odours are presented separately to the two 

 nostrils, by means of two tubes for instance, the effect is not always 

 the same. Sometimes an oscillation of sensation similar to that 

 spoken of in binocular vision ( 800) takes place. At other times, 

 the particular result depending on the nature of the odours, one 

 sensation only is felt, the one sensation wholly destroys the other. 

 And we may infer from this that when, as frequently happens, in 

 a mixture of odours we can only recognize one dominant odour, the 

 suppression of the missing sensations is not due to the chemical 

 action of one odour upon another, or to the one odour preventing 

 the other from acting on the olfactory cells ; but from a central 

 cerebral obliteration of all the sensations but one. 



861. As in the cases of the previous senses, we project our 

 olfactory sensations into the external world ; the smell appears to 

 be not in our nose, but somewhere outside us. We can judge of 

 the position of the odour however even less definitely than we can 

 of that of a sound. Our chief guide seems to be that we by turning 

 the head ascertain in which direction we experience the strongest 

 sensations. 



The sense of smell seems to play a far more important part in 

 the lives of the lower animals than it does in our own life ; and what 

 we now possess is probably the mere remnant of a once powerful 

 mechanism. We may perhaps connect with this on the one hand 

 the fact that, even in ourselves, the olfactory fibres have allotted 

 to them what is virtually a whole segment of the brain, namely 

 the olfactory lobe, and on the other hand the fact that olfactory 

 sensations seem to have an unusually direct path to the inner 

 working of the central nervous system. Mental associations 

 cluster more strongly round sensations of smell than round 

 almost any other impressions we receive from without. And 

 powerful reflex effects are very frequent, many people fainting 

 in consequence of the contact of a few odorous particles with 

 their olfactory cells. 



The assertion that the olfactory nerve is the nerve of smell has 

 been disputed. Cases have been recorded of persons who appeared 



