SENSE OF HEARING. 879 



carnivora. It is bounded internally by the tympanic membrane and its 

 function is to conduct to the middle ear the undulations collected by 

 the auricle. In the external auditory canal are found the ceruminous 

 glands secreting the wax, -which is principally intended for lubricating 

 purposes, and thus facilitates the transmission of sound, while, also, by 

 its extremely bitter taste it perhaps prevents the entrance of insects. 



The tympanic membrane forms the boundary between the external 

 auditory canal and the middle ear. This membrane consists of three 

 distinct layers, the outer, the external integument, the middle or fibrous 

 layer, and the inner or mucous layer, continuous with mucous membrane 

 lining the middle ear. This membrane is an elastic, almost unyielding, 



Ax 



Pro. 401.— Left Tympanum and Auditory Ossicles. (Lnndois.) 



A.O., externa.] meatus : M. membrana tympani, which is attached to the handle of the malleus, n, 

 and near it the short process, p i h, h«ad of the malleus ; a, incus; K. its short process, with its ligament; 

 1, long process; s, Sylvian ossicle : S. stapes: A x, A x, tho axis of rotation of the ossicles, shown in per- 

 spective ; t, line of traction of the tensor tympani. The other arrows show the movements of the ossicles 

 when the tensor contracts. 



membrane, elliptical in form, and is placed obliquely in the floor of the 

 external meatus at an angle of about 40°, being directed from above 

 downward and inward. This oblique position enables a larger surface 

 to be presented to the undulations conducted from the external auditory 

 canal than if it were placed vertically. This membrane is not entirely 

 flat, but at its centre is drawn slightly inward where the handle of the 

 malleus is attached to it, while the short process of the malleus causes 

 a slight bulging of the membrane near its upper margin (Figs. 399 and 

 400). 



The middle ear, or the drum of the ear or the tympanum, is bounded 

 by the tympanic membrane on the exterior, and on the interior by the 



