SENSE OF HEARING. 



899 



XI. we have a very interesting demonstration of the purpose and action of the 

 tympanum, in the more perfect forms of the auditory apparatus. We are now 

 prepared to inquire, in somewhat more of detail, into the actions of the differ- 

 ent parts of this apparatus j and it will be better to commence with those of 

 the Middle and Internal Ear, the accessory organs being afterwards considered. 

 902. The Membrana Tympani consists of three layers : an external one, con- 

 tinuous with the cutis of the external meatus, and consisting of dermoid tissue 

 with a covering of epidermic cells } an internal one, which is extremely thin, 

 continuous in like manner with the mucous membrane lining the tympanic 



Fig. 220. 



Diagram of the inner wall of the tympanum after maceration, the outer wall and ossicles being removed 

 a. Fenestra ovalis. b. Fenestra rotunda, c. Promontory, d. Pyramid, with the orifice at its apex. e. Pro- 

 jection of the aqueductus Fallopii. /. Some of the mastoid cells communicating with the tympanum, g. 

 Processus cochleariformis, bounding i, the canal for the tensor tympani muscle : the anterior pyramid is 

 broken off, if it existed, h. Commencement of the Eustachian tube. j. Jugular fossa, immediately below the 

 tympanum, k, k. Carotid canal, with the artery in outline, to show its course in relation to the tympanum 

 and Euatachian tube. I. Portio dura of the seventh pair of nerves, as it would be seen in the terminal part of 

 the aqueduct of Fallopius. TO. Chorda tympani, leaving the portio dura, and entering a short canal, which 

 opens in the tympanum, at the base of the pyramid, n. Grooves for the tympanic plexus. 



cavity, and also composed of dermoid tissue and epithelium; and a middle layer, 

 which, according to the recent researches of Mr. Toynbee, 1 may be separated 

 into two distinct laminae, whose fibres run in contrary directions, those of the 

 external layer (which is the stronger of the two) radiating from the malleus to wards 

 the peripheral ring to which they are attached, whilst those of the internal are 

 annular. The fibres of which these laminae are composed do not appear to be 

 muscular ; nor do they present the longitudinal parallel wavy lines character- 

 istic of ordinary fibrous membranes ; and they are rendered opaque by acetic 

 acid. Hence, although those laminae appear to be derived, the external from 

 the periosteum of the meatus, and the internal from that of the tympanic 'cavity, 

 they differ from it in elementary structure, and seem to have more in common 

 with the elastic tissue. Mr. Toynbee points out the existence of a tubular 

 ligament, inclosing the tendon of the tensor tympani muscle ; and considers 

 that the membrane is maintained by this ligament in a state of moderate tension, 

 the assistance of the muscle being only required to augment this. The function 



1 " Philosophical Transactions," 1851. 



