The Skull as a Whole. 



131 



S.m. 



-ZY 



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 

 (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 



Fig. 60. Dorsal surface of the skull: F, frontal; 

 I, interparietal; L, lacrimal; M, maxilla; MS, 

 mastoid portion of petrosal (petromastoid) ; . N, 

 nasal; P, parietal; SO, supraoccipital (squamous 

 portion of occipital); SQ, squamosal; ZY, 

 zygomatic. 



f.mx., maxillary fossa; f.t., temporal fossa; 

 l.n.s., siiperior 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 p.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. 



