THE EAR. 463 



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. 



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

 the auditory organ ; and carefully pick away with scalpel and 

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

 organ can be clearly seen. 



a. The anterior vertical semicircular canal lies in the 



natural position of the parts, close alongside the 

 posterior border of the optic lobe and the side of 

 the cerebellum. # 



b. The posterior vertical semicircular canal lies almost 



immediately below the anterior vertical canal. 



c. The horizontal semicircular canal lies opposite the 



lower border of the optic lobe and the hinder end 

 of the cerebellum. 



The three semicircular canals, as in other 

 vertebrates, are in planes, each at right angles to 

 the other two. 



