Hand atlas of human anatomy - EducationNest

Bones of the Skull. 9
A thick process extending domiward, the processus mastoideus (for the attachment
of the mm. sternocleidomastoideus, splenius capitis, longissimus capitis) presents on its medial
surface, a smooth depression, the incisura mastoidea (0. T. digastric fossa) for the attachment
of the digastric mnscle, and, further medianward, a groove, the sulcus arteriae occipitalis
(for the a. o(-cipitalis). The medial surface of the pars mastoidea is fused in fi-ont with the
pars petrosa ; behind, it is free and there presents a deep groove, the sulcus sigmoidevs (0. T.
fossa sigmoidea) (for the sinus transversus). Inside the process are mam' cavities hned by
mucous membrane, the cellulae mastoideae (cf. Fig. 11).
The pars petrosa (pyramis) is a three-sided pyramid, the base of which is directed
lateralward, and is united with the pars mastoidea. It presents for examination three sur-
faces, a fades posterior, a fades inferior and a fades anterior; at the jimctions of the
surfaces are the anguhis 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 fiUed by the fihrocartilago basalis.
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-
beUi), below by the angulus posterior; the latter is rough and unites with the lateral margin
of the pars basUaris and the pans lateralis of the occipital bone ffissura and synchondrosis
petrooccipitalis) , 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 internus, leads into the short meatus acusticus
intei-nus (for the a. and vv. auditivae internae ; nn. facialis and acusticus). Above and lateral-
ward close to the superior angle hes 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 Kg. 15), is usually only shghtly 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 sHt-shaped, opening from a dorsolateral direction, and from it a
gi-oove 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
therefi'om by a ft-ee margin, the angulus anterior, which unites with the margin of the large
wing of the sphenoid bone situated medianward from the spina angidaris, 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 oon'esponding 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. glossophar}Tigei , gangl.
jugulare n. vagi). Just in fi-ont of this notch hes the broail 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 th(! r. auricularis n. vagi).
Just medianward from the jugular fossa near the posterior angle, is seen a funnel-shaped
depression, with the apertura externa canaliculi cochleae (for the v. canahculi cochleae).