650 



THE SENSES 



others. It is thought that this forms at least an aid in determining the 

 direction whence a sound comes. 



The Middle Ear or Tympanum. The middle ear, or tympanum, 3, 

 figure 430, is separated by the membrana tympani from the external auditory 

 canal. It is a cavity in the temporal bone, opening through its anterior and 

 inner wall into the Eustachian tube. 



The Eustachian canal establishes communication between the tym anic 

 cavity and pharynx, thus equalizing the air pressure on the sides of the 

 tympanic membrane, serving the same mechanical purpose as the vent-hole 

 in a snare or bass drum. The cavity of the tympanum communicates pos- 

 teriorly with air cavities, the mastoid cells, in the mastoid process of the tem- 



Recessus epitympanic 

 Body of inc 



Short process of inc 

 Ligament of inc 



Chorda tympani nerve 



Pyramid, with tendon 



of stapedius muscle 



issuing from it 



Foot of stape 



Superfoinigament of malleus 



Head of malleus 



ior ligament of malleus 

 Handle of malleus 



Tensor tympani muscle 



Processus 



cochleariformis 

 Osseous part of 

 Eustachian tube 



FIG. 432. Left Membrana Tympani and Chain of Tympanic Ossicles (Seen from Inner 



Aspect). (Cunningham.) 



poral bone; but its only opening to the external air is through the Eustachian 

 tube. The cavity of the tympanum is lined with mucous membrane, the 

 epithelium of which is ciliated and continuous with that of the pharynx. It 

 contains a chain of small bones, ossicula auditus, which extends from the 

 membrana tympani to the fenestra ovalis. 



The Membrana Tympani. The tympanic membrane is placed in a slant- 

 ing direction at the bottom of the external canal, its plane being at an angle 

 of about forty-five degrees with the lower wall of the canal. It is formed 

 chiefly of a tough and tense fibrous membrane, the edges of which are set 

 in a bony groove. Its outer surface is covered by a continuation of the 

 epithelial lining of the auditory canal, its inner surface with part of the 

 mucous membrane of the middle ear. 



The Tympanic Ossicles. The ear bones, or ossicles, are named the 

 malleus, incus, and stapes. The malleus is attached by a long slightly curved 

 process, called its handle, to the membrana tympani, the line of attachment 



