Chap, v.] TASTE AND SMELL. 1027 



guide to connect the characters of an olfactory sensation with 

 the chemical constitution of the body giving rise to it. 



The sensation takes some time to develope after the contact 

 of the stimulus with the olfactory membrane, and may last 

 very long. When the stimulus is repeated the sensation very 

 soon dies out: the sensory terminal organs speedily become 

 exhausted. The larger, apparently, the surface of olfactory 

 membrane employed, the more intense the sensation; animals 

 with acute scent have a proportionately large area of olfactory 

 membrane. The greater the quantity of odoriferous material 

 brought to the membrane, the more intense the sensation up to 

 a certain limit; and an olfactometer for measuring olfactory 

 sensations has been constructed, the measurements being given 

 by the size of the superficial area, impregnated with an odorif- 

 erous 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 ex- 

 haustion. The minimum quantity of material required to pro- 

 duce an olfactory sensation ma} T 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 sen- 

 sation similar to that spoken of in binocular vision (§ 602) 

 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. 



§ 638. As in the cases of the previous senses, we project 

 our olfactory sensations into the external world; the smell ap- 

 pears to be not in our nose, but somewhere outside us. We 

 can judge of the position of the odour however even less defi- 

 nitely 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 



