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tympanum is placed, as the only part of the external auditory tube, 

 directly subservient to the function of hearing, and that the rest is 

 merely to protect the auditory canal from the pressure of the 

 condyle of the lower jaw; which, was it not defended by the bony 

 sheath, would be in constant danger of being compressed, and its 

 area obliterated, whenever the inferior maxillary bone was exten- 

 sively moved. In the earlier periods of life, when the condyles have 

 not the power of extensive motion, the auditory tube does not exist ; its 

 place being occupied by a fibrous membrane, which is attached, by 

 one end, to the edge of the curved bone that forms the rudiments of 

 the auditory tube, and by the other, to the cartilaginous portion of the 

 auditory canal.* 



The groove of the auditory tube, in which the circumference of 

 the membrane of the tympanum is encased, and which consequently 

 regulates the line of direction of this membrane, is, when first observed, 

 nearly horizontal. By degrees, it becomes so inclined from without, 

 downwards and inwards, that at birth it is very oblique. Hence, 

 as Bichat has observed, the membrane of the tympanum is, at that 

 period, continuous and almost parallel to the superior side of the 

 auditory canal ; while the inferior side forms, with the membrane of 

 the tympanum, a very acute angle, corresponding to the extremity of 

 the canal. After birth, the direction of the groove becomes more 

 vertical, and this alteration is accompanied by a corresponding one 

 in the membrane of the tympanum. It may be here observed, that, 

 as the diameter of the groove, in which the membrane of the tym- 

 panum is encased, is at birth nearly as large as it will ever be, so is 



the membrane itself. 



y2 



* I here, for the sake of description, confine the term andilory canal to the tube,which con. 

 veys the air to the membrane of the tympanum ; and wish to designate, by the words auditory 

 tube, the portion of bone which enters into tiie formation of the auditory canal. 



