1168 SURFACE AND SURGICAL ANATOMY. 
anteriorly to form the roof of the osseous portion of the Eustachian tube, while posteriorly 
it roofs over the mastoid antrum. Externally the tegmen is limited by the petro- 
squamous suture, which may remain unossified for some years after birth, thus 
affording a channel along which pyogenic infection may spread from the middle ear to the 
meninges and brain. Infection may also spread along the small veins which convey blood 
from the tympanum to the e superior petrosal and lateral sinuses. 
The floor of the tympanum is formed mainly by the bone forming the jugular fossa, 
which is occupied by the bulb of the internal jugular vein. When the lateral sinus is 
large and unusu: lly f far forward the bulb is likewise large, and the fossa, which is con- 
sequently deeper, may arch up into the floor of the tympanic cavity, from which it may 
be separated merely by a thin and translucent plate of bone which occasionally shows an 
osseous deficiency. In cases where this condition existed the jugular bulb has been 
wounded in the operation of paracentesis of the tympanic membrane. 
Anteriorly the tympanic cavity leads into the Eustachian tube, which brings it into 
communication with the naso-pharynx. In the child the Eustachian tube is shorter, 
Roof of mastoid antrum 
Middle cranial fossa Mastoid antrum 
Groove for lateral sinus 
Groove for 
middle menin-~ 
geal artery > 
Head of 
Portion of external semi- 
malleus 
circular canal 
Body of ineus— 
—~_Elevation caused by 
y “< 
Canal for tensor tympani muscle ~ ™ aqueduct of Fallopius 
: ae / 
Carotid canal / 
d 
Meibrana tympani / 
Styloid process 
Facial nerve 
Posterior margin of jugular foramen 
Fic. 796.—SECTION THROUGH PETROUS PORTION OF TEMPORAL BONE OF ADULT. 
Showing the relation of the middle ear to the middle and posterior fosse of the skull. 
wider, and more horizontal than in the adult, hence inflammations are more lable to 
spread along it to the tympanum. 
Above the level of the membrana tympani is the tympanic attic or epitympanic space, 
which communicates posteriorly by means of a triangular opening (ad/tus ad antrum) 
with the mastoid antrum; the base of the triangle, directed upwards, is formed by the 
tegmen tympani ; its apex, directed downwards, is formed by the meeting of the inner and 
outer walls. The opening will admit an instrument half acm. in diameter. The tympanic 
attic contains from before backwards the head of the malleus, the body and short process 
of the incus, the latter projecting backwards into the aditus. When these structures 
are covered with inflamed mucous membrane or granulations, drainage from the 
mastoid antrum into the tympanum proper is interfered with. The boundaries of the 
aditus, important surgically, are as follows: superiorly, the tegmen tympani ; internally, an 
eminence of compact bone, containing the external semicircular canal, below and in front of 
which is a second smaller prominence, corresponding to that portion of the aqueduct of 
Fallopius which curves immediately above and behind the foramen ovale. The wall of the 
aqueduct is here thin and not infrequently deficient, in which case inflammation may readily 
spread from the tympanum to the facial nerve. The outer wall of the aditus is formed by 
the deepest part of the upper and outer wall of the osseous external auditory canal. 
The posterior wall of the tympanum, below the aditus ad antram, is formed by dipldic 
bone which contains the descending portion of the aqueduct of Fallopius. 
a 
4 
* 
i aa 
