CHAPTER XX 

 OUTPOSTS OF THE INTELLIGENCE SERVICE 



(6) DISTANCE RECEPTOR FOR SOUND 



THE EAR 



" A clue to the structure of a machine lies in the discovery of the purpose for 

 which it was designed and the manner in which its various parts are co-ordinated 

 to secure that end. That is eminently true of the ear." Keith. 



The ear is a modified touch receptor. In the lower invertebrates 

 it consists of hair-Hke appendages, either on the free surface or 

 in a depression, more or less protected. In the higher vertebrates it 

 is a nmch more complicated structure. The human organ of hearing 

 may be considered as composed of three structural elements, viz : 



External ear — collector and conductor of sound to the middle ear. 



Middle ear — converter of air vibrations to a to-and-fro move- 

 ment of a hinged piston-like lever and the accentuation of 

 these movements. 



Part of internal ear — transformer of mechanical pressure, via 

 hydraulic pressure, into touch stimulus. 



1. External ear. The structure of this presents no outstanding 

 points of physical interest. It consists of the pinna and the 

 external acoustic meatus at the end of which is the membrana 

 tympani or eardrum (Fig. 63). 



(a) The pinna is a flattened horn presenting irregularities of 

 surface. If these undulations are filled in with wax or if the pinna 

 is awanting, the quality of sounds is altered and difficulty in 

 localising sound is increased. This may be due to a differential 

 reflection of tones by the pinna, e.g. it may reflect a fundamental 

 tone more strongly than the partial or vice versa. 



(b) The external acoustic meatus is a curved tube about 21-26 

 mm. long. Its function is twofold, (i.) On account of its shape, 

 secretion, and hairs (at orifice) it protects the delicate tympanic 

 membrane from draughts, dust, and from the incursion of insects, 

 and maintains an equable temperature. These are its main 

 functions, (ii.) The sound waves are conducted by reflection from 

 the walls without loss of intensity, and directed almost per- 

 pendicularly on to the drum which lies at an angle of 150° to the 

 axis of the canal. 



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