THE INTERNAL EAR 319 



the temporal bone, containing ossicles for the conduction of 

 sound waves to the internal ear. The cavity communicates, 

 through the Eustachian tube, with the pharynx, and this is its 

 oniy direct connection with the external air, though it does com- 

 municate 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 ex- 

 ternally by skin and internally by mucous membrane. 



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 tym- 

 pani, and articulates by its head with the incus. The incus has 

 the shape of an anvil; its base articulates 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 laxa- 

 tor tympani are attached to the neck of the malleus; the sta- 

 pedius to the neck of the stapes. These bones constitute a 

 chain, which conveys 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 surrounding 

 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. 



The bony labyrinth consists of the vestibule, cochlea and 

 semicircular canals. The vestibule occupies the mid-portion 

 of the labyrinth, and is that part with which the middle ear com- 

 municates by the fenestra ovalis; it communicates also with the 

 cochlea and semicircular canals, and on its internal aspect are 

 openings for the entrance of some of the branches of the audi- 

 tory nerve. The cochlea, shaped like a snail shell, runs off 



