SECTION II 

 STRUCTURE OF AUDITORY APPARATUS 



EXTERNAL EAR 



The external ear consists of two parts: (1) that external to the skull 

 called the pinna and (2) that internal, called the meatus. The pinna in v 

 animals is a horn-shaped structure which is provided with two sets of muscles. 

 The immediate response to a slight sound is a pricking of the ears by means 

 of the intrinsic muscles, and the directing of the orifices towards the source of 

 sound, through the action of the extrinsic muscles. The functions of the 

 pinna are firstly to ascertain approximately the direction of the source of 

 sound, and secondly to concentrate the sound waves into the meatus. It 

 may also be said to have a third function, namely to protect the internal struc- 

 tures; the stiff hairs with which it is provided must prevent to a considerable 

 extent the entrance of foreign bodies. 



In man the pinna is undergoing retrogression ; not only has it lost its 

 trumpet shape but also it has become almost entirely immobilised from disuse. 

 It is improbable therefore in man that pinna has any power of accentu- 

 ating sound waves ; this is borne out by experiments in which the undulations 

 are filled with wax, and by cases in which the pinna has been cut off. 



The form of the pinna in man may have a slight influence in the judgment of the 

 direction from which sounds proceed. It has been noticed that a compound tone 

 changes slightly in quality as its position in relation to the ear is altered. This is partly 

 due to the fact that the auricle may reflect a fundamental tone more strongly than 

 the partial or the converse. According to Rayleigh this difference in quality is deter- 

 mined chiefly by the fact that diffraction of the sound waves occurs as they pass round 

 the head to the ear remote from the source of the sound, so that the partial tones reach 

 the two ears in different degrees of intensity and determine a difference in quality of 

 the sound as heard by the two ears. 



THE EXTERNAL AUDITORY MEATUS in man is about one inch long and 

 directed forwards, inwards, and slightly upwards. Its general function, other 

 than as a mere conductor of the sound waves, is to protect the delicate vibrat- 

 ing membrana tympani which closes its inner end. This it does partly because 

 of its narrow tubular shape and partly owing to its considerable curvature. 

 Opening on the skin of the meatus are special sebaceous glands which secrete 

 a yellow wax (cerumen) with bitter taste and peculiar odour. The wax not 

 only protects the cuticle of the ear and the membrana tympani from drying 

 but, together with the hairs at the orifice of the meatus, serves to repel 

 insects and prevent their entering. By the length of the meatus moreover 

 the drum is protected from draughts and its temperature is maintained 

 constant. 



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