THE EAR 427 



c. The columella is a small rod of bone and cartilage, 



lying on the inner side of the tympanic mem- 

 brane, but readily seen through it. It lies al- 

 most horizontally, its free anterior end formiag 

 a slight projection in the centre of the mem- 

 brane. 



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 up the tympanic cavity. 



d. The tympanic cavity is the middle, dilated portion 



of the tympano-Eustachian passage. On its 

 inner wall is the fenestra ovaUs, 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 front of the transverse ridge forming the 

 hinder border of the palate. 



2. The auditory organ. 



The essential organ of hearing is imbedded in 

 the side-wall of the skuU. It is, however, easy to 

 dissect, as the layer of bone immediately investing 

 it is very firm and compact, and so preserves the 

 shape of the organ, while the rest of the bone is 

 soft and spongy, and easily removed. 



Bemove the hinder part of the side of the skull, containing^ 

 the auditory organ; and carefully pick away with scalpel and 



