250 HUMAN PHYSIOLOGY. 



ear and the labyrinth. It is narrow from side to side, but rela- 

 tively long in its vertical and anteroposterior diameters ; it is sep- 

 arated from the external auditory canal by a membrane the mem- 

 brana tympani ; from the internal ear it is separated by an osseo- 

 membranous partition, which forms a common wall for both cavities. 

 The middle ear communicates posteriorly with the mastoid cells ; 

 anteriorly with the nasopharynx, by means of the Eustachian tube. 

 The interior of this cavity is lined by mucous membrane continuous 

 with that lining the pharynx. 



The membrana tympani is a thin, translucent, nearly circular mem- 

 brane, measuring about ^i of an inch in diameter, placed at the inner 

 termination of the external auditory canal. The membrane is in- 

 closed within a ring of bone, which in the fetal condition can be 

 easily removed, but in the adult condition becomes consolidated with 

 the surrounding bone. The membrana tympani consists primarily of 

 a layer of fibrous tissue, arranged both circularly and radially, and 

 forms the membrana propria; externally it is covered by a thin layer 

 of skin continuous with that lining the auditory canal ; internally 

 it is covered by a thin mucous membrane. The tympanic membrane 

 is placed obliquely at the bottom of the auditory canal, inclining at 

 an angle of forty-five degrees, being directed from behind and above 

 downward and inward. On its external surface this membrane pre- 

 sents a funnel-shaped depression, the sides of which are somewhat 

 convex. 



The Ear Bones. Running across the tympanic cavity and forming 

 an irregular line of joined levers is a chain of bones which articulate 

 with one another at their extremities. They are known as the 

 malleus, incus, and stapes. 



The form and position of these bones are shown in figure 34. 



The malleus consists of a head, neck, and handle, of which the 

 latter is attached to the inner surface of the membrana tympani ; the 

 incus, or anvil bone, presents a concave, articular surface, which re- 

 ceives the head of the malleus ; the stapes, or stirrup bone, articulates 

 externally with the long process of the incus, and internally, by its 

 oval base, with the edges of the foramen ovale. 



The tensor tympani muscle consists of a fleshy, tapering portion, 

 Y* of an inch in length, which terminates in a slender tendon ; it 

 arises from the cartilaginous portion of the Eustachian tube and the 

 adjacent surface of the sphenoid bone. From this origin the muscle 



