508 



THE AUDITORY APPARATUS 



the floor are longer than the posterior wall and the roof, 

 respectively. Moreover, the diameter of the canal is not 

 uniform. It is narrowest at the isthmus, which lies about 

 5 mm. from the tympanic membrane ; and its vertical diameter 

 is greatest at the lateral end, whilst its antero-posterior 

 diameter is greatest at its medial end. These facts must 

 be borne in mind during the removal of foreign bodies which 

 have made their way into the canal. As the tube passes from 

 the surface, medially, it describes a gentle sigmoid curve, but 

 its general direction is towards the median plane with a 



Osseous part of meatus 



Recessus 



epitympanicus 



Malleu 



Cochlea 



Tympanum 



Membrana 



tympani 



Internal carotid 

 artery 



.Crus antihelicis 

 inferior 



Cyraba conchae 

 rus helicis 



Cartilaginous 

 part of meatus 

 'Cavum concha; 



Lower boundary 

 of incisura 

 intertragica 



FIG. 207. Vertical transverse section through the Right Ear : anterior half 

 of section viewed from behind. (Howden.) 



slight inclination forwards. The skin lining the cartilaginous 

 portion is abundantly furnished with ceruminous glands and 

 is provided also with laterally directed hairs, which tend to 

 prevent the entrance of dust. The cutaneous lining of the 

 osseous part, which is thin and tightly adherent to the 

 subjacent periosteum, is destitute of hairs, and glands are for 

 the most part absent. The cutaneous lining of the meatus is 

 continued, in the form of an exceedingly delicate layer, over 

 the lateral surface of the membrana tympani. 



When the direction, the length, and the diameters of the 

 external meatus have been noted, the dissectors should 

 examine the lateral surface of the tympanic membrane, 



