Bones of the Skull. 9 



A thick process extending downward, the processus mastoideus (for the attachment of 

 the mm. sternocleidomastoideus , splenius capitis, longissimus capitis) (see Figs. 7, 9 and 59) 

 presents on its medial surface, a smooth depression, the incisura mastoidea (0. T. digastric 

 fossa) for the attachment of the digastric muscle, and, further medianward, a groove, the sulcus 

 arteriae occipitalis (for the a. occipitalis). The medial surface of the pars mastoidea (see 

 Figs. 8 and 9) is fused in front with the pars petrosa; behind, it is free and there presents 

 a deep groove , the sulcus sigmoideus (0. T. fossa sigmoidea) (for the sinus transversus). 

 Inside the process are many cavities lined by mucous membrane, the cellulac mastoideae 

 (cf. Fig. 11). 



The pars petrosa (pyrami*) (see Figs. 711, 59, 60, 6366) is a three-sided 

 pyramid, the base of which is directed lateral ward, and is united with the pars mastoidea. 

 It presents for examination three surfaces, a fades posterior, a fades inferior and a fades 

 anterior; at the junctions of the surfaces are the angulus posterior, the angulus anterior, 

 and the angulus superior. The tip, or apex pyramidis, is directed medianward and forward, 

 and is inserted in the angle between the occipital bone and the sphenoid bone, leaving, 

 however, an irregular chink, the foramen lacerum which is filled by the fibrocartilago basalts. 



The fades posterior pyramidis almost vertical, is bounded above by the angulus 

 superior with the sulcus petrosus superior (for the sinus petrosus superior; tentorium cere- 

 belli), below by the angulus posterior ; the latter is rough and unites with the lateral margin 

 of the pars basilaris and the pars lateralis of the occipital bone (fissura and synchondrosis 

 peirooccipitalis), and contains above at the upper surface of this place of union the sulcus 

 petrosus inferior (for the sinus petrosus inferior). At about the middle of the posterior sur- 

 face a circular opening, the porus acusticus inter nus , leads into the short meatus acusticus 

 internus (for the a. and vv. auditivae internae ; nn. facialis and acusticus). Above and lateral- 

 ward close to the superior angle lies the fossa subarcuata which, running below the superior 

 semicircular canal, receives a process of the dura mater; this fossa, very well marked in the 

 new-born (see Fig. 15), is usually only slightly indicated in the adult. Still further lateral- 

 ward and downward is seen the apertura externa aquaeductus vestibuli (for the ductus endo- 

 lymphaticus) ; it is usually slit -shaped, opening from a dorsolateral direction, and from it a 

 groove runs downward to the incisura jugularis. 



The fades inferior pyramidis is placed horizontally and is very uneven and 

 rough. In front its boundary is formed laterally by the pars tympanica and medianward 

 therefrom by a free margin, the angulus anterior, which unites with the margin of the large 

 wing of the sphenoid bone situated medianward from the spina angularis, to form the fissura 

 and synchondrosis sphenopetrosa. Behind it is bounded by the angulus posterior. A notch, 

 sometimes scarcely noticeable, the incisura jugularis, close to the small processus intrajugu- 

 laris, forms with the corresponding incisura jugularis of the occipital bone (p. 2), the foramen 

 jugulare (lateral compartment for the v. jugularis interna; medial for the sinus petrosus in- 

 ferior; nn. glossopharyngeus , vagus, accessorius, gangl. superius n. glossopharyngei , gangl. 

 jugulare n. vagi). Just in front of the lateral compartment of this notch lies the broad fossa 

 jugularis (for the bulbus v. jugularis superior) with a small hole, one of the openings of the 

 canaliculus mastoideus, and a groove leading to this, the sulcus canaliculi mastoidei (for the 

 r. auricularis n. vagi). Just medianward from the jugular fossa near the medial compartment 

 of the incisura jugularis, is seen a funnel-shaped depression, with the apertura externa 

 canaliculi cochleae (for the v. canaliculi cochleae). 



