706 THE ORGANS OF SENSE. 
seen below the last; it 1s caused by the upward and forward prolongation of the 
styloid process. 
The anterior wall (paries carotica) is narrowed in its transverse diameter by 
the approximation of the outer and inner boundaries of the cavity, and in its 
vertical diameter by the descent of the roof and ascent of the floor. It presents 
(Fig. 515) two parallel canals, one above the other, separated by a thin lamella of 
bone, the processus cochleariformis (septum canalis musculo- tubarii). The higher 
eridicmeiler of the two is termed the canal for the tensor tympani muscle (semicanalis 
m. tensoris tympani) and hes immediately below the tegmen tympani. It hasa 
diameter of about 2 mim. and extends on to the inner wall of the tympanic cavity. 
The lower and larger canal gradually increases in size from before backwards, and 
forms the bony part of the Eustachian tube (semicanalis tube auditive). It opens 
on the anterior wall of the tympanic cavity opposite the orifice leading into the 
mastoid antrum. The Eustachian tube is described on p. 708. 
The outer wall (paries membranacea) is formed almost. entirely by the meim- 
brana tympani, which closes the inner extremity of the external auditory meatus 
(Fig. 516) and is fixed, throughout the greater part of its circumference, in a groove, 
the sulcus tympanicus. ‘The bony ring containing this sulcus is deficient superiorly 
where it exhibits a distinct notch, the notch of Rivinus. On a level with the upper 
edge of the membrane, and in front of the ring of bone in which it is fixed, is the 
inner end of the Glaserian fissure, or remnant of the fissura petro-tympanica. ‘This 
transmits the tympanic branch of the internal maxillary artery and lodges the 
processus gracilis and anterior ligament of the malleus. Close to the inner 
extremity of the fissure is the canal of Huguier, or iter chorde anterius, through 
which the chorda tympani nerve leaves the tympanum. 
Membrana Tympani—This is an elliptical disc, its greatest diameter, 9 to 
10 mmn., being directed from above and behind, downwards and forwards, whilet 
its least diameter is from 8 to 9 mm. _ Its antero-inferior portion inclines markedly 
inwards, and thus the membrane is placed very obliquely, forming an angle of 
about 55° with the lower and anterior walls of the external auditory meatus ; its 
antero-inferior part is, therefore, most distant from the outer orifice of the meatus. 
The membrane is said to be more oblique in cretins and deaf mutes, and more per- 
pendicular in musicians. 
The circumference of that portion of the membrane which is fixed in the sulcus 
tympanicus is considerably thickened, and is named the annulus fibro-cartilagineus. 
It is prolonged from the anterior and posterior extremities of the notch of Rivinus 
the short process of the malleus in the form of two ligamentous bands, the 
anterior and posterior malleolar folds or ligaments (plica malleolaris anterior et 
posterior). The small triangular portion of the membrane (Fig. 516), situated above 
these folds, is thin and lax, and constitutes the pars flaccida or membrane of 
Shrapnell; the main portion of the membrane is, on the other hand, tightly 
stretched and termed the pars tensa. A small orifice, sometimes seen in the pars 
flaccida, is probably either a pathological condition or has been artificially pro- 
duced during manipulation. The handle of the malleus is firmly fixed to the inner 
surface of the membrana tympani and draws its central portion inwards, rendering 
its outer aspect concave. The deepest part of this concavity corresponds with the 
lower extremity of the handle of the malleus, and is named the umbo membrane 
tympane or navel. 
The membrana tympani consists of three layers: (1) external, integumentary 
(stratum cutaneum); (2) middle, fibrous (membrana propria); (3) internal, mucous 
(stratum mucosum). 
The external layer (stratuin cutaneum) is continuous with the integumentary 
lining of the meatus and consists of a thin layer of cutis, covered by epidermis. 
The cutis is thickest near the circumference; the epidermis, on the other hand, is 
thickest near the centre of the membrane. 
The middle layer (membrana propria) consists of two sets of fibres: (a) external 
or radial (stratum radiatum), immediately under the integument, and radiating 
outwards from the handle of the malleus to the annulus fibro-cartilagineus ; 
(b) internal or circular (stratum cireulare), the fibres of which are numerous 
