Middle Ear 97 



the membrane may be evenly struck on each side ; at the same 

 time also he sets the tensor tympani in action so as to steady the 

 membrane ; otherwise the explosion may rend it. 



The stapedius arises in a small pyramid of bone upon the inner 

 wall of the tympanum, and is inserted into the neck of the stapes. It 

 is supplied by a tympanic branch of the facial nerve. Its action is to 

 drive the plate of the stirrup farther into the oval foramen, and so to 

 compress the fluid in the vestibule. 



The tympanum and the mastoid cells are lined by a delicate 

 mucous membrane, which is continuous with that of the pharynx 

 through the Eustachian tube. It covers the ossicles and the nerves, 

 and forms the inner layers of the membrana tympani. Its epithelium 

 is of the columnar ciliated variety. 



Supply of tympanum. Arterial twigs come from the internal 

 carotid, stylo-mastoid, internal maxillary, and middle meningeal. The 

 tympanic veins end in the petrosal and lateral sinuses, and in the 

 internal jugular. The nerve-supply is from Jacobson's branch of 

 the glosso-pharyngeal (p. 69) ; but Arnold's nerve also helps with 

 a delicate twig. The chorda tympani wraps itself in mucous mem- 

 brane as it hurries through the tympanum, passing between the handle 

 of the hammer and the long process of the incus, but it gives no 

 branch to the cavity. The lymphatics descend to glands behind the 

 angle of the jaw, and massage over them, downwards from the mastoid 

 process, may give much help in emptying mucus from the middle ear. 



The membrana tympani is stretched obliquely at the bottom of 

 the external auditory meatus. In the adult its border is fixed in a 

 groove in the bone, but in the child to the tympanic ring. The ring 

 being deficient above, the attachment there is less firm only to 

 the periosteum and thus it may be unglued by a box-on-the-ear, or 

 by blood or pus escaping from the tympanum. (Tillaux.) 



The membrane is composed of circular and radiating fibres, which 

 are covered on one side by a thin layer of skin from the external 

 auditory meatus, and in the other by the mucous lining of the middle 

 ear. Between this mucous membrane and the fibrous layer the handle 

 of the malleus descends as far as the centre, to which part it is 

 attached, drawing it slightly inwards at a sort of umbilicus. Its 

 arteries are derived from the tympanic branch of the internal maxil- 

 lary, and from the stylo-mastoid of the posterior auricular, which 

 ramify respectively upon the lower and upper parts. The auriculo- 

 temporal nerve endows it with sensibility. 



Paracentesis of the tympanum may be required for the evacua- 

 tion of abscess from the middle ear ; also for the transmission of waves 

 of sound through a membrane which has become thickened and stiff 

 by chronic inflammation the auditory nerve being known to be 

 healthy (p. 102). In this case it is often impossible to maintain the 

 desirable patency of the opening, whereas after the opening of a tym- 



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