54 SPECIAL ANATOMY. 



b. Internal surface, at the superior part a distinct horizontal sulcus arte- 

 rialis. 



c. Circumference. Three-fourths of a circle, free, above, upright ; the in- 

 ternal border behind cut off obliquely, where it unites with the cranial bone; 

 before united with the Sphenoid. 



2. Pars Mastoidea, nipple-like portion behind the squamous, and deeper, 

 separated from it above by a deep incision, incisura parietalis ; below by the 

 petrous portion (with the external auditory meatus). In the thickness of the 

 bone are placed the sinus s. celluke mastoidece, into which we may pass from 

 the cavity of the tympanum. They are covered with delicate periosteum and 

 pavement epithelium, not mucous membrane. 



a. External surface, convex, rough, presents below and before the proc. 

 mastoideus (for the muse, sternocleido-mast., spknius capitis, retrahentes auri- 

 cula) ; hence inwards, indsura mastoidea (ibr the muse, digastricus) ; still 

 deeper and parallel a small incision (for muse, trachefo-mastoid.) ; backwards, 

 for. mastoideum (for emissarium Santotini), and sometimes Art. meningea pos- 

 teriora. 



b. Internal surface, concave, with the broadest portion of the fossa sig- 

 moidea (continuation of the sulcus transversus'), the internal opening of the 

 for am. mastoideum. 



c. Circumference very thick; dentated; anteriorly, united in the pointed 

 Incisura parietalia with the parietal; behind, thick and uneven, with the 

 occipital bone. 



3. Pars Petrosa, s. os petrosum, petrous bone, pyramidal, trilateral, lies 

 obliquely from without inwards and from behind forwards, looking with the 

 base outwards, with the apex towards the cranial cavity. Very solid, but 

 brittle ; it encloses the organ of hearing. 



a. Basis, between proc. zygomatic. and mastoid. presents Meatus auditorius 

 externus, the external auditory canal, behind the cavit. glenoidalis, oval, in the 

 centre narrower than at its extremities (in the foetus, instead of it, a ring, 

 annulus tympani). 



b. Apex, obtuse, the exit of the Canalis caroticus and part of the foram. 

 lacerum anterius- externally, tuba Eustachii; above that, canal, tensoris tym- 

 pani. 



c. Surfaces. Inferior, from without to within; Processus styloideus, twelve 

 to fifteen lines long, sometimes moveable, always in animals; backwards, 

 foramen stylo-mastoideum (for facial nerve and stylo-mastoid artery) , the inferior 

 opening of the Canal. Fallopice. Inwards from the styloid process; fossa 



jugularis (for the bulbus Ven. jugularis} ; the anterior half of the foram. jugular. 

 s. lacei'um posterius (for the Nervi glossopharyngeus, vagus et accessorius, ram. 

 jugularis Nervi sympathies et Vena jugularis interna), with the Vallecula s. fos~ 

 sula petrosa (for the Ganglion petrosum, Nervi glossopharyngei); in which the 

 inferior opening of the Can. tympanicus ; before the fossa jugularis the en- 

 trance to the Canalis Caro'icus (for the Carotis interna et ramus caroticus nerv. 

 sympathici) ; [from the Carotid Canal, foramina lead into the Tympanum.] 

 Upon the septum, between fossa jugularis et canal, caroticus, the narrow en- 

 trance to the Aqu&ductus Cochlea. 



d. Anterior (internal) surface. This is the posterior wall of the central 

 cranial fossa, with Hiatus Canalis Fallnpia (for the Nerv. Vidianus superfi- 

 cialis) in the centre ; anteriorly and externally to this the superior opening 

 of the canal, tympanicus; posteriorly, eminentia arcuata of the superior semi- 

 circular canal of the Labyrinth of the Qar. A groove (for the sinus pctrosus 



