THE EAE. 457 



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



lying on the inner side of the tympanic mem- 

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

 most horizontal, its free anterior end forming 

 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. 



Ctd 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 

 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 skull. 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. 



Bcmove the hinder part of the side of the skull, containiiuj 

 the auditory organ; and carefully pick away lolbh scalpel and 



