838 A MANUAL OF PHYSIOLOGY 



obliquely downwards, and at the same time somewhat forwards, so 

 that if prolonged the membranes of the two ears would cut each 

 other in front of, and also below, the horizontal line passing through 

 the centre of each (Figs. 322, 323). 



The tympanum contains a chain of little bones stretching right 

 across it from outer to inner wall. Of these the malleus, or hammer, 

 is the most external. Its manubrium, or handle, is inserted into 

 the membrana tympani, which is not stretched taut within its bony 

 ring, but bulges inwards at the centre, where the handle of the 

 malleus is attached. The stapes, or stirrup, is the most internal of 

 the chain of ossicles, and is inserted by its foot-plate into a small 



FIG. 323. TYMPANUM OP LEFT EAR, SHOWING THE OSSICLES (MORRIS). 



i, superior, and 4, external ligament of malleus ; 2, head ; 7, short process, and 10, 

 manubrium or handle of malleus ; 5, long process of incus, terminating in 9, the os 

 orbiculare; 6, base, and 8, head of stapes; n, Eustachian tube ; 12, external auditory 

 meatus ; 13, membrana tympani ; 3, upper, and 14, lower part of tympanum. 



oval opening the foramen ovale on the inner wall of the tympanic 

 cavity. A membranous ring the orbicular membrane surrounds 

 the foot of the stapes, helping to fill up the foramen and attaching 

 the bone to its edges. The incus, or anvil, forms a link between the 

 malleus and the stapes. The auditory ossicles, as well as the whole 

 cavity of the tympanum, are covered by pavement epithelium. The 

 tympanum is not an absolutely closed chamber ; it has one channel 

 of communication with the external air the Eustachian tube. By 

 the action of the cilia which line this tube the scanty secretion of the 

 middle ear is moved towards its pharyngeal opening. The loosely- 

 jointed chain of ossicles is steadied and its movements directed by 

 ligaments and by the tension of its terminal membranes. It forms 



