456 MEMBRANA TYMPANI. 



The Membrana Tympani, 



Appears to be fixed, in a bony ring in the meatus auditorius, 

 and is nearly circular in form. It is not perpendicular to the 

 meatus, but has an oblique position in it ; the inferior margin 

 projecting farther inwards than the superior. 



The external surface forms a conical concavity, being appa- 

 rently drawn in by the malteus, one of the small bones of the 

 ear, to which it is attached.* 



It is asserted by several anatomists, that this membrane 

 maybe separated into four lamina, viz. 1. The Cuticle; and, 

 2. The Cutis ; which are continued from the skin of the 

 meatus externus. 3. The proper Membrani Tympani', and 

 4. The Lining Membrane of the Tympanum; which is 

 extended over the internal surface of the. membrana tympani. 

 Notwithstanding these lamina, it is almost transparent in the 

 living subject when in a healthy state, and appears highly 

 polished on its external surface, when light is thrown upon it. 

 It has been injected so as to appear vascular in every part ; 

 and one or two vessels are sometimes seen even in common 

 injected preparations. In some cases of inflammation it has 

 been uniformly red. 



It was formerly asserted by Rivinus, a professor at Leipsic, 

 that there was a natural aperture in this membrane ; but 

 although this opinion has had several votaries, it is certainly 

 erroneous. At the same time it is to be observed, that an 

 aperture occasioned by accident or disease, is sometimes per- 

 ceived in persons who enjoy the faculty of hearing to a consid- 

 erable degree. Some persons, thus circumstanced, are accus- 

 tomed to force tobacco smoke from the mouth through the ears. 



It is asserted by a respectable anatomist f that the membrana 



* In birds, it is always found convex outwards. This is a universal mark of 

 distinction in the membrana tympani of birds and mammalia, the two classes 

 of animals only, in which it exists as a thin vibrating membrane. P. 



f See Mr. Home's Memoir on the structure and uses of the membrana tym- 

 pani in the Philosophical Transactions for 1800. 



