166 HUMAN PHYSIOLOGY. 



shape of the pinna is concave and presents a little below the centre a deep 

 depression, the concha. The external auditory canal extends from the 

 concha inward for a distance of about one and a quarter inches. It is 

 directed somewhat forward and upward, passing over a convexity of bone, 

 and then dips downward to its termination ; it is composed of both bone 

 and cartilage and lined by a reflection of the skin covering the pinna. At 

 the external portion of the canal the skin contains a number of tubular 

 glands, the ceruminous glands, which in their conformation resemble the 

 perspiratory glands. They secrete the cerumen or ear wax. 



The Middle Ear or Tympanum is an irregularly shaped cavity hollowed 

 out of the temporal bone and situated between the external ear and the 

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

 and antero- posterior 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 mastoid 

 cells, anteriorly with the naso-pharynx 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 two-fifths of an inch in diameter, placed at the inner ter- 

 mination of the external auditory canal. The membrane is enclosed within 

 a ring of bone which, in the foetal 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 45, 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 jointed levers, is a chain of bones which articulate with 

 each other at their extremities. They are known as the malleus, incus and 

 stapes. 



The form and position of these bones are shown in Fig. 24. 



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 



