EXTERNAL AND MIDDLE PORTIONS OF THE EAR 



765 



by two muscles, known as the muse, tensor tympani and the muse, 

 stapedius. 



The Tympanic Membrane or Eardrum. The tympanic membrane 

 is stretched across a cartilaginous ring which is placed obliquely in 

 the inner end of the auditory canal. It possesses a somewhat oval 

 shape and is tilted at an angle of 40 in a direction from above and with- 

 out to a point within and below, this peculiarity in its position en- 

 abling it to present a much larger surface to the sound waves. The 

 membrane itself is 9.5-10 mm. in length and 8 mm. in breadth. Its 

 thickness measures 0.1 mm. and its area 50 mm. It consists of three 

 layers, its middle coat being formed of fibrous tissue which is en- 

 veloped externally by a delicate layer of skin, and internally by the 

 mucous membrane, lining the general cavity of the tympanum. The 

 fibers of the median coat are chiefly arranged in a radial direction, 



Membrana flaccida Posterior ligament 



Anterior ligament 



Long process of incus 



End of mamtbrium of malleus 



FIG. 383. MEMBRANA TYMPANI, AS SEEN WITH THE OTOSCOPE. (Heusman.) 



but some of them are also adjusted circularly around its center. The 

 latter are especially numerous in the region where this membrane is 

 joined to the ring of cartilage. 



The inner surface of the eardrum lies in contact with the handle, 

 or manubrium, of the first ossicle, commonly known as the hammer- 

 bone or malleus. This process is securely fastened to its median 

 layer, the membrana propria, by an overlapping of its circular fibers. 

 When observed through the external meatus, the line of contact be- 

 tween the malleus and the eardrum is sharply outlined by an opaque 

 ridge which commences near its upper anterior margin and extends 

 downward and backward to a point slightly below its center. The 

 surface of the eardrum is not flat, but convex toward the outside. Its 

 apex points inward, this central depression, or umbo, being caused by 

 the inward traction of the tip of the manubrium. It will be seen, 

 therefore, that the different radial fibers uniting this process with the 

 membrane, are arranged as arches around a common center. 



