MEMBRANA TYMPANI. 197 



the tympanic membrane from impressions that might other- 

 wise be too violent. 



Structure of the Membrana Tympani. The general ar- 

 rangement of the membrana tympani has already been de- 

 scribed in connection with the topographical anatomy of the 

 auditory apparatus. 1 This structure, which is of great impor- 

 tance in the physiology of hearing, is delicate, elastic, about 

 the thickness of ordinary gold-beater's skin, and is subject to 

 various degrees of tension, from the action of the muscles of 

 the middle ear and different conditions of atmospheric press- 

 ure within and without the cavity of the tympanum. Its 

 form is nearly circular. From a number of accurate meas- 

 urements of its diameter in the adult, by Sappey, we may as- 

 sume that its ring measures a little more than f of an inch 

 vertically and about f of an inch antero-posteriorly. The ex- 

 cess of the vertical over the horizontal diameter is about -fa 

 of an inch. 3 Notwithstanding the assertion of some of the 

 older anatomists, that the tympanic membrane presents one 

 or two small perforations, it is now almost universally re- 

 garded as forming a complete division, without openings, be- 

 tween the external meatus and the middle ear ; or, if any 

 openings exist, they are exceedingly minute. 



The periphery of the tympanic membrane is received into 

 a little ring of bone, which may be separated by maceration 

 in early life, but which is consolidated with the adjacent 

 bony structures in the adult. This bony ring is incomplete 

 at its superior portion, but, aside from this, resembles the 

 groove which receives the crystal of a watch. At the periph- 

 ery of the membrane, is a ring of condensed fibrous tissue, 

 which is received into the bony ring. This ring also pre- 

 sents a break at its superior portion. 



The concavity of the membrana tympani presents out- 



1 See p. 155. 



2 SAPPEY, TraitecTanatomie, Paris, 1871, p. 801. The above measurements 

 are given, assuming that the position of the membrane is vertical. 



