THE TEMPORAL BONE. 



179 



the interior of the cranium is very thin. Behind the .t^lenoid fossa the horizontal 

 part of the sr|ii;imosal forms tlic roof of the external au(htf)ry meatus. 



The Tympanic Portion.' — The tympanic portion of the temporal bone 

 appears as a trumpet-shaped layer of bone, forming all but the roof of the external 

 auditory meatus. Its edge is tliiii in front, thick below, and very thin t>ehind, where 

 it curls up before the mastoid lo meet the postavu'irular jirocess of the scjuamosal. 

 It is separated from the mastoid by the minute lynif^ano-iiiastoid fissure. Tiie ante- 

 rior part of the tympanic portion, called the tympanic plaic, runs obliquely forward, 

 concealing the petrosal. It is separated from the glenoid fossa and from the thick 

 anterior edge of the squamosal hy the fissure of Ulaser, which opens intc) the tym- 

 panic cavity. The outer end of the hssure is closed ; the inner pari is double, since 

 a thin piece of the petrous, the tcgmen tympani, bends down between the squamous 

 and tympanic portions. The lower edge of the tympanic plate ends free. A part 

 covering the base of the styloid [jrocess is the vagina/ process,'^ which sometimes 



splits to enclose it. t- a 



^ Fio. T98. 



SQUAMOUS PORTION 



ZyKomn 



Hustarbian tube 



Articular eminence 



Glcnoifl fof.sa 

 Tegmen tympaiii 

 Glaserian fissurt 



TYMPANIC PORTION 



Styloid process' 



Stylo-masloid foramen 

 Mastoid process 



Digastric groove 



Carotid canal 



-A(|uaductus cochlete 

 "'4 I, PETRO-MA8TOID PORTION 



W r>. ■'■W"--''^ Jugular fossa 



Joining occipital 



Occipital groove 

 Right temporal bone from below 



The Petro-Mastoid Portion. ■• -This part of the temporal bone may for 

 convenience of description be subdivided into the mastoid and the petrous. The 

 mastoid subdivision forms a [)art of the wall of the skull behind the tymf»anic. 

 It is prolonged downward into a nipple-shaped process, the outside of which is rough 

 and slightly prominent. On its lower surface, under cover of the apex, is the 

 digastric groove' for the origin of the posterior belly of the digastric muscle. Just 

 internal to this, at the very edg(- f)f the bone, is the much smaller occipital groove 

 for the occipital artery. The ridge between the tw(j may be developed intf> a para- 

 mastoid process. The greater part of the internal surface is occupied by a broad and 

 deep^r<7(7Z'^,' running obliquely downward, forward, and inward for the lateral sinus 

 on its way to the jugular foramen. The direction of this groove is very uncertain. 

 Sometimes it descends gradually ; at othc^rs it turns far forward and descends nearly 

 vertically. In the latter case it approaches closer than otherwise to the outer wall 

 of the skull, but the distance in all cases is very variable (P'igs. 199, 200). It 

 may be only a few millimetres. As it descends it reaches the inner side of the 

 antrum and the mastoid cells. It is separated frf>m the antrum by a plate some six 



> Pars tympanica. ^ Vagina proccusus »tyloldeu». ' Pari petrosa et mastoldia, * Incisora mastoldea. '' Sulcus »ignioid(ni». 



