32O THE SENSES 



inward and slightly forward from the concha to terminate 

 at the membrana tympani, or drum. Its inner part is in the 

 petrous portion of the temporal bone; its external part is 

 fibro-cartilaginous in structure. The whole is lined by in- 

 tegument. 



The Middle Ear (Tympanum). This is a cavity at the 

 bottom of the external auditory canal in the petrous portion 

 of the temporal bone, containing ossicles for the conduction 

 of sound waves to the internal ear. The cavity communi- 

 cates, through the Eustachian tube, with the pharynx, and 

 this is its only direct connection with the external air, though 

 it does communicate with the mastoid air cells. It is lined by 

 mucous membrane. The membrana tympani, separating it 

 from the external auditory canal, is fibrous in structure. It 

 is lined externally by skin and internally by mucous mem- 

 brane. 



The three ossicles of the middle ear are the malleus, incus 

 and stapes. The malleus, shaped like a hammer, is attached 

 in a vertical direction to the upper radius of the 

 membrana tympani, and articulates by its head with the 

 incus. The incus has the shape of an anvil ; its base articu- 

 lates with the malleus, while its small extremity curves 

 downward to articulate with the neck of the stapes. The 

 base of the stapes is applied to the membrane covering the 

 fenestra ovalis. The tensor and laxator tympani are at- 

 tached to the neck of the malleus ; the stapedius to the neck 

 of the stapes. These bones constitute a chain, which con- 

 veys the vibrations of the membrana tympani to the fenestra 

 ovalis. 



The Internal Ear (Labyrinth). This consists of a series 

 of cavities in the petrous portion of the temporal bone lined 

 by a peculiar membrane. When the bony substance sur- 

 rounding these cavities is carefully removed it is found 

 that that portion immediately outside them is harder than 

 the adjacent structure. This constitutes the bony labyrinth, 

 while the membrane inside the bony walls is the membranous 

 labyrinth. 



