224 HUMAN PHYSIOLOGY. 



labyrinth. It is narrow from side to side, but relatively long in its vertical 

 and anteroposterior diameters; it is separated from the external auditory 

 canal by a membrane the membrana 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 mas- 

 toid 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 membrane, 

 measuring about f of an inch in diameter, placed at the inner termination 

 of the external auditory canal. The membrane is inclosed 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 circu- 

 larly and radially, and forms the membrana propria; externally it is covered 

 by a thin layer of skin continuous with that lining the auditory canal; inter- 

 nally 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 presents 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 36. 



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 receives the head of the 

 malleus; the stapes, or stirrup bone, articulates externally with the long proc- 

 ess of the incus, and internally, by its oval base, with the edges of the fora- 

 men ovale. 



The tensor tympani muscle consists of a fleshy, tapering portion, \ 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 passes nearly horizontally back- 

 ward to the tympanic cavity; just opposite to the fenestra ovalis its tendon 

 bends at a right angle over the processus cochleariformis, and then passes 

 outward across the cavity, to be inserted into the angle of the malleus near 

 the neck. 



