708 THE ORGANS OF SENSE. 
tympanic cavity, to end at the stylo-mastoid foramen. (4) The processus cochleari- 
formis, or septum canalis musculo-tubari. This process extends backwards, above 
the anterior end of the fenestra ovalis, where it makes a sharp outward curve and 
forms a pulley, over which the tendon of the tensor tympani muscle plays. (5) A 
funnel-shaped recess, the fossula fenestre cochlee, which is situated behind and 
below the promontory and almost hidden by its overhanging edge. It leads for- 
wards, upwards, and inwards to an irregularly-oval opening, termed the fenestra 
rotunda seu cochlex, which in the macerated bone communicates with the cochlea, 
but in the recent state is closed by a membrane, the membrana tympani secundaria. 
This membrane appears angularly bent along a line joining its antero-inferior two- 
thirds with the postero-superior third. It consists of three layers: (a) external, 
continuous with the mucous lining of the tympanum and containing a network 
of capillaries; (b) middle, or substantia propria, the fibres of which radiate chietly 
towards the periphery of the membrane—some branched, dendritic fibres are also 
present; (c) internal, continuous with the epithelial lining of the labyrinth. 
(6) Between the fenestra ovalis above and the fossula rotunda below is a small 
circular depression, the sinus tympani, perforated by one or two minute foramina 
for blood-vessels, whilst immediately internal to it is the ampullated extremity of 
the posterior semicircular canal. 
Mastorip ANTRUM AND MAStToID AIR-CELLS. 
The mastoid antrum (antrum mastoideum seu tympanicum) is seen in the tem- 
poral bone at birth as a cavity, having a vertical measurement of 7-9 mm. and a 
transverse of 9-11 mm., and is nearly as large in the newly-born child as in the 
adult. Rooted in by the tegmen tympani, its floor and inner wall are formed by 
the petro-mastoid, while externally it is closed by the junction of the thin outer 
part of the squama with the pars mastoidea. At ‘birth its outer wall has a thick- 
ness of only 1-2mm., but by the ninth year this has increased to about 10 mm. 
Coincident with the growth of the mastoid process the mastoid air-cells are de- 
veloped downwards and backwards as diverticula from the amie and present 
the greatest possible variations in different skulls. They may be large, compara- 
tively few in number, and involve the whole process, in which case the compact 
bone which surrounds them is extremely thin and the innermost cells are only 
separated by a transparent lamella from the lateral sinus. In other cases the 
cells may be small and numerous, only invading a portion of the process, the 
remainder consisting of diploétic tissue. No definite conclusion Gan be come to as 
to their condition by e external percussion or examination. A solid process is occa- 
sionally seen. The air-cells are not limited to the mastoid process, but extend 
forwards over the roof of the meatus, upwards towards the squama, and inwards 
towards the temporo-occipital suture, whilst in a few cases they are seen to invade 
the pars jugularis of the occipital bone. They are lined by thin mucous membrane 
continuous with that of the tympanum; the deep surface of the mucous membrane 
is firmly fixed to the periosteum, while its free surface is covered by a layer of 
flattened, non-ciliated epithelium. 
EUSTACHIAN TUBE. 
The Eustachian tube (tuba auditiva Eustachi1) leads from the tympanic cavity to 
the naso-pharynx and transmits air to the former, in order that the pressure on the 
inner and outer surfaces of the membrana tympani may be equalised; it may also 
serve to convey mucous secretion away from the tympanic cavity. Its outer 
extremity, the ostium tympanicum tube auditive (Fig. 515), opens into the anterior 
part of the tympanic cavity below the canal for the tensor tympani muscle. 
Directed downwards and inwards, the tube ends on the upper part of the naso- 
pharynx by a wide orifice, the ostium pharyngeum tube auditive (Fig. 490). It 
measures about an inch and a half (56 mm.) in length, and forms with the hori- 
zontal plane an angle of 30° to 40°, with the sagittal plane an angle of about 45°, and 
with the bony part of the external meatus one of 135° to 140°. It consists of two 
portions: (@) an antero-internal, fibro-cartilaginous part, the pars cartilaginea tube 
