462 DISSECTION OF THE PIGEON. 



a. The external auditory meatus is the outer part of 



this passage, from the exterior to the tympanic 

 membrane. The external aperture is a circular 

 hole on the side of the head, behind and below 

 the posterior border of the orbit. 



Cut away the lower jaw. Cut away the external meatus 

 carefully with scissors, so as to expose the tympanic mem- 

 brane. 



b. The tympanic membrane is a thin transparent 



septum separating the external meatus from the 

 tympanic cavity. 



c. The columella is a small rod of bone and carti- 



lage lying on the inner side of the tympanic 

 membrane, but easily seen through it. It lies 

 almost horizontal, its free anterior end forming 

 a slight projection in the centre of the membrane. 

 The columella is formed from the uppermost 

 part of the cartilage of the hyoidean arch and 

 probably corresponds to part, or the whole, of the 

 hyo-mandibular cartilage of the dog-fish. 



Cut across the tympanic membrane in front of the colu- 

 mella, to open the tympanic cavity. 



d. The tympanic cavity is the middle dilated portion 



of the tympano-Eustachian passage.^ On its 

 inner wall is the fenestra ovalis, into which the 

 posterior end of the columella is inserted. 



e. The Eustachian tube is the innermost division of 



the tympano-Eustachian passage. It is a narrow 

 tube, leading from the anterior and lower angle of 

 the tympanic cavity, and running forwards and 

 inwards between the proper base of the skull and 

 the basi-temporal bone. The Eustachian tubes 

 of the two sides unite, and open into the roof of 

 the mouth by a median aperture, just behind the 

 posterior border of the narial opening, and in 



