THE BONES OF THE SKULL. 147 
external acoustic meatus. The boundary between the two is 
indicated externally by a shallow oblique groove, the position of 
which indicates roughly that of the tympanic membrane within. 
The medioventral margin of the bone is articulated with the basi- 
occipital, but the swollen portion is separated from the latter by a 
broad groove terminating posteriorly in the jugular fossa and the 
jugular foramen. Immediately in front of the jugular fossa, the 
rounded aperture of the external carotid foramen, transmitting 
in the natural condition the internal carotid artery, leads into the 
carotid canal of the interior 
of tympanic portion. At the 
anterior end of the groove, 
communicating with the fora- 
men lacerum, is the anterior 
opening of the carotid canal, 
the internal carotid foramen, 
and on its lateral side the much 
larger aperture of the auditory 
(Eustachian) tube. The rela- 
tions of these apertures are seen 
to best advantage’ when the 
petrotympanic is disarticulated Bea De oe ee Ce eas ee ear 
from, the associated posterior  2uditory complex of the left’side x3. The 
lateral portions of the tympanic bulla and 
sphenoid bone. The auditory external acoustic meatus have been removed, 
exposing the structures of the tympanic 
tube iS then seen to lead directly cavity. MS, mastoid portion; P, petrous 
portion; T, tympanic portion (bulla tympani). 
into the tympanic cavity. oN c.m., mastoid cells; c.t., tympanic cavity; 
f.c., cochlear fenestra; in., incus; m.a.e., 
fine bristle may be passed external acoustic meatus; m.m., manubrium 
of the malleus; m.so., supraoccipital’ margin 
through the carotid canal from of petromastoid; p.m., mastoid process; st., 
stapes; t.a., aperture of auditory tube. 

one foramen to the other. 
6. THE STRUCTURES OF THE TYMPANIC CAVITY 
The relations of the tympanic cavity and associated structures 
may be studied with advantage in a skull from which the lateral 
wall of the tympanic bulla and external acoustic meatus has been 
removed, the surface displayed being as indicated in Fig. 64. 
The tympanum or middle ear is enclosed by the tympanic and 
petromastoid portions of the temporal complex. The attached 
