CHAPTER XIX 

 OUTPOSTS OF THE INTELLIGENCE SERVICE 



(b) 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-like appendages, either on the free surface or 

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

 it is a much more complicated structure. The human ear 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 piston-like lever and the accentuation of these 

 movements. 



Part of internal ear transformer of mechanical pressure, via 

 hydraulic pressure, into nervous energy. 



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. 43). 



(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 two-fold, (i) On account of its shape, 

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

 membrane from draughts and dust, and from the incursion of 



200 



