THE SKULL AS A WHOLE. 
131 
ridge, the temporal line (linea temporalis), the latter forming 
also the lateral margin of the roof of the skull in this region. 
Behind the posterior root of the zygomatic arch, the external 
surface of the lateral wall is largely occupied by the swollen tym- 
panic bulla (bulla tympani), 
formed by the tympanic bone. 
It contains the capacious 
tympanic cavity (cavum 
tympani) and certain skeletal 
structures of the middle ear, 
namely, the auditory ossi- 
cles (ossicula auditus), the 
relations of which are more 
fully dealt with below (p. 147). 
The dorsal portion of the 
tympanic bulla is continuous 
with a short bony tube which 
opens at a short distance 
dorsally by a large oval aper- 
ture. This tube is part of a 
more extensive canal, the 
external acoustic meatus 
(meatus acusticus. externus) 
which, in the natural condi- 
tion, leads downward through 
the base of the external ear to 
the tympanic membrane. 
The tympanic bulla does not 
form the lateral wall of the 
skull in this region, and is not 
exposed to the cranial cavity. 
It is applied closely to the 
external surface of the perio- 
tic or petromastoid bone 

poe. 
Dorsal surface of the skull: F, frontal; 
Fic. 60. 
I,interparietal; L, lacrimal; M, maxilla; MS, 
mastoid portion of petrosal (petromastoid); N, 
nasal; P, parietal; SO, supraoccipital (squamous 
portion of occipital); SO, squamosal; JESS 
zygomatic. 
f.mx., maxillary fossa; f.t., temporal fossa; 
l.n.s., superior nuchal line; l.t., temporal line; 
p.f., frontal process of premaxilla; p.mx., 
maxillary process of frontal; p.o.e., external 
occipital protuberance; p.s.a. and ).s.p., 
anterior and posterior supraorbital processes of 
frontal; p.sc., subcutaneous process of lacrimal; 
p.z., zygomatic process of squamosal; p.z.m., 
zygomatic process of maxilla; s.f., frontal spine; 
s.m., spina masseterica. 
(os petrosum), which forms the lateral boundary of the cranial 
cavity, and contains the structures of the internal ear. 
The external 
or mastoid portion of this bone appears in the space enclosed 
between the tympanic bulla and the jugular process of the occipital 
