CAVITY OF THE TYMPANUM. 455 



The cavity of the tympanum is lined by a membrane which 

 has been considered as similar to the periosteum : but it is as- 

 serted by Bichat that this membrane, when inflamed, resembles 

 the mucous membranes ; and that in its natural state it secretes 

 a mucus which passes into the fauces by the Eustachian tube. 



This linino- membrane is continued over the internal surface 



O 



of the membrana tympani, and was supposed by the aforesaid 

 author to be reflected so as to cover the small bones of the ear. 



The opinion of Bichat respecting the nature of the mem- 

 brane, is rendered probable by analogical reasoning, viz. The 

 internal surface of the cavity must be in want of mucus or 

 cuticle, as it is in contact with the air. The membrane is very 

 sensible in a sound state, and therefore is very different from 

 periosteum, which it was supposed to resemble. 



The tympanum or middle ear, is but an accessory part of 

 the organ of hearing, the use of which is to augment and trans- 

 mit sound received through the medium of the ear. It is found 

 only in mammalia, birds, and reptiles. The whole organ of 

 hearing in fishes, consists merely in a membranous bulb, placed 

 at the exterior, and upon which is ramified the auditory nerve. 

 The most simple idea that can be formed of the tympanum, 

 (as seen in its development in the human foetus and in inferior 

 animals,) is, that of a cylindrical prolongation of the mucous 

 membrane of the pharynx, to the front part of the labyrinth, 

 where it is dilated to form the cavity of the drum, and contract- 

 ed on the side of the pharynx to form the Eustachian tube. A 

 bony shell is subsequently formed around the drum, and the 

 tympanic half of the Eustachian tube. The closure of the 

 tympanum by bone, is not complete. The membrana tympani 

 forms part of the wall on the side of the meatus. The internal 

 wall of the cavity of the tympanum, is formed by the bony 

 labyrinth. The mucous membrane reflected from the pharynx 

 into the cavity of the tympanum, is extremely thin, delicate and 

 very vascular and receives many filaments of nerves. It lines 

 the whole circumference of the cavity, as well as the mastoid 

 cells communicating with it, and is reflected over the chain of 

 bones, and subserves the part of capsular ligament in holding 

 them together at their place of articulation. 



