MIDDLE EAR. 



365 



FIG. 106. 



longer than the superior. Its circumference is fitted like 

 a watch-glass into the circular furrow in the base of 

 the meatus, in the adult, and into the tympanic ring of 

 the foetus. It 

 is a thin, semi- 

 transparent, 

 dry, parch- 

 ment-like mem- 

 brane, and con- 

 sists of three 

 layers, an ex- 

 ternal or cuta- 

 neous, already 

 described, a 

 middle, or fi- 

 brous, and an 

 internal or mu- 

 cous. 



The middle layer forms the membrane proper of the 

 the drum, and gives the form and strength to this septum 

 between the meatus and tympanum. It is fibrous, and the 

 fibres are seen radiating from the circumference to the 

 centre, where the malleus is attached. Mr. E. Home, from 

 the examination of this membrane in an elephant, was led 

 to regard its structure as muscular, which opinion, how- 

 ever, is not generally adopted. The internal layer is the 

 continuation of the mucous membrane from the pharynx 

 into the Eustachian tube and the cavity of the tympanum. 

 The membrana tympani is capable of injection and presents 

 a beautiful arrangement of radiated and delicate vessels. 

 It is formed for vibrating, and its function is to receive the 

 sonorous undulations from the air, and transmit them to the 

 chain of little bones, in the tympanum, connected with it. 



The internal boundary of the tympanum presents a bony 



FIG. 106 represents the external, middle, and internal Ear. a External 

 ear. & Meatus auditorius externus. c Membrana tympani. d Malleus, e 

 Incus. / Cavity of the tympanum, g Stapes, h Semi-circular canals, t 

 Auditory nerve, j Cochlea, k Eustachian tube. 



