EXTERNAL EAR 



507 



end and the cartilaginous part of the external acoustic meatus 

 to its lateral border. The dissector should now cut away the 

 tragus of the auricle, to expose the orifice of the external meatus 

 which lies at the bottom of the concha ; then, with knife or 

 scissors, he must remove the anterior wall of the cartilaginous 

 part of the external meatus. Next pass a probe into the bony 

 part of the meatus to gauge its length, and, whilst the probe is 

 kept in position as a guide, cut away the anterior wall of the 

 bony part of the meatus, taking care not to injure the tympanic 

 membrane which closes the medial end of the meatus. When 

 the dissection is completed the boundaries of the meatus 

 and the lateral surface of the tympanic membrane should be 

 examined. 



Tympanic antrum Canalis facialis 



Apertura tympanica 

 canaliculi chordae 



=- Cochlea 



Canal for tympanic nerve 



FIG. 206. Frontal section of the Right Temporal Bone passing through the 

 external and the internal acoustic meatuses. 



Meatus Acusticus Externus. The external acoustic meatus 

 runs forwards and medially, from its lateral orifice to its 

 medial boundary, and, during its course, it forms a slight curve 

 with the convexity upwards. Its total length, measured from 

 the bottom of the concha to the tympanic membrane, is about 

 24 mm., of which 8 mm. corresponds with the cartilaginous 

 part, and 16 mm. with the bony part of the canal : but, as the 

 membrana tympani is placed obliquely, the anterior wall and 



