540 OF SENSATION. 



we are suffering under an ordinary cold. The highest part of the nasal 

 fossae appears to be that, in which there is the most acute sensibility to 

 odours; and hence it is, that, when we snuff the air, so. as to direct it 

 into this portion of the cavity, we perceive delicate odours, which would 

 otherwise have escaped us. .The acuteness of the sense of Smell depends, 

 in no small degree, upon the extent of surface exposed by the membrane 

 lining the nasal cavity ; and in this respect Man is far surpassed by 

 many of the lower Mammalia, especially the Ruminants, which are 

 warned by its means of the proximity of their enemies. The habit of 

 attention to Sensory impressions of this class; however, very much 

 heightens their acuteness : hence in those who suffer under blindness and 

 deafness conjointly, it is usually the principal means by which indivi- 

 duals are distinguished, and the presence of strangers recognised; and 

 there are cases, in which individuals in a state of somnambulism have 

 exhibited a degree of acuteness of smell, quite comparable to that which 

 is characteristic of Deer, Antelopes, &c. 



948. Besides ministering to the sense of Smell, by stimulating the 

 secreting powers of its surface, the Fifth pair has another very impor- 

 tant function, that of endowing the interior of the nose with common 

 sensibility, and thus receiving the impression produced by acrid or pun- 

 gent substances, which act upon it in the same way as they do upon the 

 tongue. Such substances are felt, by the irritation they produce, rather 

 than smelt ; and the sensation they occasion gives rise to the consensual 

 act of sneezing, by which a violent blast of air is directed through the 

 nasal passages, in such a manner as to clear them of the irritating mat- 

 ter, whether solid (as snuff), fluid, or gaseous. Hence this action may 

 be excited by the contact of an irritant with the Schneiderian mem- 

 brane, after the olfactory nerve has been divided, if the branches of the 

 fifth pair be entire ; whilst it does not take place when the fifth pair is- 

 paralysed, even though the sense of smell is retained. 



5. Of the Sense of Hearing. 



949. By this sense we become acquainted with the sounds produced 

 by bodies in a certain state of vibration; the vibrations being propagated 

 through the surrounding medium, by the corresponding waves or undu- 

 lations which they produce in it. Although air is the usual medium 

 through which sound is propagated, yet liquids or solids may answer the 

 same purpose. On the other hand, no sound can be propagated through 

 a perfect vacuum. It is a fact of much importance, in regard to the 

 action of the Organ of Hearing, that sonorous vibrations which have 

 been excited, and are being transmitted, in a medium of one kind, are 

 not imparted with the same 'readiness to others. The following conclu- 

 sions have been drawn from 'experimental inquiries on this subject. 



I. Vibrations excited in solid bodies, may be transmitted to water 

 without much loss of their intensity ; although not with the same readi- 

 ness that they would be communicated to another solid. 



II. On the other hand, vibrations excited in water lose something of 

 their intensity in being propagated to solids ; but they are returned, as 

 it were, by these solids to the liquid, so that the sound is more loudly 



