546 THE AUDITORY APPARATUS 



THE AUDITORY APPARATUS, 



THE organ of hearing admits of a very natural subdivision 

 into three parts, viz., the external, the middle, and the 

 internal ear. The external ear consists of the auricle and 

 the external acustic meatus. The auricle collects the waves 

 of sound, and is, comparatively speaking, of subsidiary im- 

 portance in man, although it is highly developed and of 

 considerable service in some of the lower animals. The 

 external acustic meatus is a passage leading inwards from 

 the bottom of the concha to the membrana tympani, which 

 separates the external from the middle ear. The middle ear 

 is a narrow chamber termed the tympanic cavity. It is inter- 

 posed between the external acustic passage and the internal 

 ear or labyrinth, and the main part of its lateral wall is formed 

 by the membrana tympani. Stretching across the cavity of 

 the tympanum, from its lateral to its medial wall, there is a 

 chain of three small bones, called the auditory ossicles. The 

 internal ear or labyrinth is a most essential part of the organ. 

 It consists of a complicated system of cavities situated in the 

 densest part of the petrous portion of the temporal bone. These 

 cavities contain fluid called perilymph, and also a membranous 

 counterpart of the bony chambers, called the membranous 

 labyrinth. Within the latter there is fluid termed endolymph. 



Dissection. The dissection of the ear should be conducted differently 

 on opposite sides. 



On one side remove the lateral pterygoid lamina and the remains of the 

 external and internal pterygoid muscles, if that has not been done already. 

 Then clear away the tensor palati muscle and expose the lateral surface of 

 the auditory tube. Dissect on the postero-medial aspect of the tube and 

 expose the levator palati muscle from the lateral side. Follow the muscle 

 downwards and medially, below the lower orifice of the tube, into the 

 soft palate. Then detach the auditory tube from the posterior border 

 of the medial pterygoid lamina ; cut the levator palati, at the point 

 where it enters the soft palate, and separate the cartilaginous part of the 

 auditory tube from any parts of the wall of the pharynx which may still be 

 connected with it. When this has been done turn to the temporal bone ; 

 place the saw at right angles to the external surface of the squamous part 

 and saw through the bone, along the line of the petro-tympanic fissure, to the 

 posterior border of the spine of the sphenoid. Turn next to the medial 

 surface and saw through the body of the sphenoid at the level of the anterior 

 boundary of the foramen lacerum ; then, with the aid of the chisel and bone 

 forceps, detach the posterior border of the great wing of the sphenoid from 



