506 THE AUDITORY APPARATUS 



some of the lower animals. The external acoustic meatus 

 is a passage leading inwards, from the bottom of the concha 

 of the auricle, to the membrana tympani, which separates 

 the external ear from the middle ear. The middle ear is 

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

 posed between the external acoustic 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 the 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. The 

 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. 



EXTERNAL EAR. 



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 veli 

 palatini muscle and expose the lateral surface of the auditory 

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

 expose the levator veli palatini 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 veli palatini 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 that 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 the anterior border of the petrous part 

 of the temporal bone. When the dissection is properly done 

 the greater part of the temporal bone is removed from the 

 remainder of the skull, with the cartilaginous part of the auditory 

 tube attached to the anterior margin of its petrous portion, and 

 a small part of the body of the sphenoid bone attached to its 

 apex. The anterior wall of the mandibular fossa was separated 

 by the first saw-cut, but the posterior wall is still present, with the 

 cartilaginous part of the auditory tube attached to its medial 



