THE EAR 415 



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 almost 

 horizontally, 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 hypidean 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 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 pos- 

 terior 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. 



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



the avMtory organ ; and carefully pick away with scalpel and 



forceps the outer spongy layer of hone, until the shape of the 



organ can be cleanly seen. 



