CH. LIH.J HEARING. 713 



affect the nerves of tactile sense (fifth nerve) more than the 

 olfactory nerves. 



The sense of'smell is excited either by gaseous or very finely- 

 divided solid particles; these affect the terminations of the 

 olfactorial or bipolar cells and the path to the brain of the 

 nervous impulse so set up we have already indicated. Liquids, 

 unless they are volatile (that is, give off vapours), do not, as a 

 rule, excite the sense ; thus Weber could not smell the slightest 

 odour when his nostrils were completely filled with water con- 

 taining eau-de-Cologne. It is matter of common experience that 

 odours and flavours (which are really odours) cannot be perceived 

 readily when the amount of moisture in the nose is increased, as 

 when one has a bad cold. 



On the other hand, the mucous membrane must not be too 



dry ; this also impairs the delicacy of the sense. The delicacy 



of the sense is most remarkable ; thus, Valentin calculates that 



- ooo of a grain of musk can be distinctly smelt ; and even 



this can be improved by practice, as in certain tribes of Indians. 



We cannot at present give a scientific classification of odours ; 

 the only possible classification into pleasant and unpleasant is a 

 matter of individual education and taste to a great extent. 



CHAPTER LIII. 



HEARING. 

 Anatomy of the Ear. 



THE Organ of Hearing is divided into three parts, (i) the 

 external, (2) the middle, and (3) the internal ear. The two first 

 are only accessory to the third or internal ear, which contains the 

 essential parts of the organ of hearing. The accompanying figure 

 shows the relation of these divisions, one to the others (fig. 536). 



External Ear. The external ear consists of the pinna and 

 the external auditory nieatus. 



The principal parts of the pinna are two prominent rims 

 enclosed one within the other (helix and antihelix), and enclosing 

 a central hollow named the concha ; in front of the concha 

 is a prominence directed backwards, the tragus, and opposite 

 to this one directed forwards, the antitrayus. From the concha, 

 the auditory canal, with a slight arch directed upwards, passes 

 inwards and a little forwards to the membrana tympani, to 

 which it thus serves to convey the vibrating air. Its outer 



