MEMBRANE OF THE TYMPANUM. 807 



and by the anterior tympanic vessels. Crossing the membrane 

 towards the upper part is the chorda tympani nerve, which issues 

 through a special aperture close to the Glaserian fissure. 



The roof (tegmen tympani ; fig. 296, tt) is a thin plate of bone The roof is 

 .-<l>u rating the tympanic cavifcy from the cranium. It occasionally perforated. 

 presents one or more apertures, where the mucous lining of the 

 tympanum comes into contact with the dura mater. 



The floor separates the tympanum from the jugular fossa (/'/), Floor is 

 and is more or less excavated by small cells, which are extensions cellular - 

 of the tympanic cavity, and lined by a prolongation of its mucous 

 membrane. 



An anterior wall is present only in the lower half of the space, in front is 

 which it separates from the carotid canal (cc) ; in the upper half is 

 the tympanic orifice of the Eustachian tube. tube. 



The posterior wall is similarly deficient in the upper half, where Behind are 

 there is a large aperture leading into a space called the antrum JJJJJJJJ? 

 mastoideum (am), from which the mastoid cells are given off. Below deum 

 this opening, but near the inner wall, and on a level with the 

 narrow part of the promontory, is the small conical projection of an(i 

 the pyramid (py). At the summit of the pyramid is a small pyramid, 

 orifice, from which a canal leads backwards and downwards to the 

 aqueduct of Fallopius : the canal lodges the stapedius muscle, with canal 

 Sometimes there is a slender round bar of bone connecting the 

 pyramid to the promontory. 



Some objects that have been referred to above, viz., the mem- 

 brana tympani, the Eustachian tube, the mastoid cells, and the 

 secondary tympanic membrane, require separate notice. 



The MEMBRAXA TYMPANI (fig. 297) is a thin translucent disc Tympanic 

 between the external auditory canal and the cavity of the tyni- m 



panum. It is rather elliptical in shape, and its longest diameter, 



which is directed from ab6Ve down, measures about two-fifths of 



an inch. By its circumference it is attached to a groove at the 



inner end of the auditory passage. In the foetus it is supported attachment ; 



by a separate osseous ring the tympanic bone (/). The mem- 



brane is placed very obliquely, so that it forms an angle of about position ; 



45 with both a horizontal and a sagittal plane, the outer surface 



looking downwards and forwards. It is concave towards the is rather 



auditory canal, being sloped inwards from the circumference to the shaped" ; 



centre ; and it projects into the cavity of the tympanum. The malleus 



handle of the malleus (one of the ossicles ; 6) is attached to 



the inner side of the membrane from the centre to the upper 



margin. 



Structure. The membrane is formed of three strata, external, internal, It consists 

 and middle. The outer one is continuous with the integuments of the meatus of acut a- 

 auditorius ; and the inner is derived from the mucous membrane of the ^mucous 

 tympanum. The middle layer is formed of fibrous tissue, and is fixed to the and a fibrous 

 groove in the bone. From its centre, where it is tirmly united to the la yer. 

 extremity of the handle of the malleus, fibres radiate towards the circum- 

 ference ; and near the margin, at the inner aspect, lies a band of stronger 

 circular fibres (fig. 297, c), which bridges across the notch at the upper part 

 of the tympanic bone. 



