MEMBRANA TYMPANI 549 



Whervthe direction, the length, and the diameters of the 

 external meatus have been noted, the dissectors should 

 examine the lateral surface of the tympanic membrane. 



Membrana Tympani. The slope of the tympanic mem- 

 brane has already been referred to. It slopes very obliquely 

 downwards, anteriorly and medially, and it is deeply concave 

 externally. The deepest point of the concavity is the umbo, 

 which corresponds with the lower end of a bar of bone, the 

 handle of the malleus, which is embedded in the membrane 

 and can be seen through the thin layer of tissue covering 

 it. The handle of the malleus extends upwards, and 

 slightly posteriorly, from the umbo towards the roof of the 

 meatus ; and a short distance from the upper margin of 

 the membrane it becomes continuous with a small laterally 

 directed process, the lateral process of the malleus, which bulges 

 the membrane towards the meatus. Above the lateral process 

 of the malleus is a portion of the membrane which is less 

 tense than the remainder. This is the membrana flacdda 

 (ShrapnelPs membrane). It is bounded anteriorly and 

 posteriorly by relatively thickened folds, the anterior and 

 posterior tympano-malleolar folds. The whole of the peripheral 

 margin of the membrane, except that which corresponds 

 with the membrana flaccida, is lodged in a ring-like sulcus 

 of bone, the annulus tympanicus, which is formed by the 

 tympanic element of the temporal bone. 



Dissection, After the examination of the external meatus is completed 

 the dissector must secure the tensor tympani muscle, which springs from the 

 anterior aspect of the petrous part of the temporal bone, close to the apex 

 and above the level of the cartilaginous part of the auditory tube. Having 

 secured it, he must trace it laterally, above the auditory tube, to the point 

 where it passes into the bony canal through which it enters the tympanum. 

 Then he must cut away the "antero-lateral wall of the cartilaginous part of 

 the auditory tube, from the pharyngeal orifice to the upper extremity, and 

 pass a probe through the bony part of the tube into the tympanum. He 

 should next turn to the anterior surface of the petrous part of the temporal 

 bone and, with chisel and bone forceps, carefully remove the legmen 

 tympani and expose the tympanic cavity from above. The dissection must 

 be carried anteriorly into the auditory tube and posteriorly into the tympanic 

 antrum. As the dissection is carried anteriorly a narrow margin of bone 

 must be left along the anterior border of the tympanic membrane, and care 

 must be taken to avoid injury to the tendon of the tensor tympani, which 

 emerges from the extremity of its bony canal, near the medial wall of the 

 tympanum, and crosses the cavity to be inserted into the malleus. The 

 chorda tympani nerve, which passes anteriorly, close to the tympanic 

 membrane and above the tendon of the tensor tympani, must also be 

 preserved if possible. 



