72 THE SKELETON 



of irregular depressions, whilst occasionally it is present as a well-marked fissure 

 (fig. 92) directed obliquely downward and forward. The mastoid portion is 

 prolonged downward behind the external acoustic meatus into a nipple-shaped 

 projection, the mastoid process, the tip of which points forward as well as down- 

 ward. The process is marked, on its medial surface, by a deep groove, the 

 mastoid notch (digastric fossa), for the origin of the digastric muscle, and 

 again medially by the occipital groove for the occipital artery. 



The outer surface is perforated by numerous foramina, one, of large size, being usually 

 situated near the posterior border and called the mastoid foramen. It transmits a vein to the 

 transverse (lateral) sinus and the mastoid branch of the occipital artery. The mastoid portion 

 gives attachment externally to the auricularis posterior {retrahens aurem) and occipitalis, and, 

 along with the mastoid process, to the sterno-mastoid, splenius capitis, and longissimus capitis 

 {trachelo-mastoid) . Projecting from the postero-superior margin of the external auditory 

 meatus there is frequently a small tubercle — the supra-meatal spine — behind which the surface 

 is depressed to form the mastoid (supra-meatal) fossa. 



The inner surface of the mastoid portion presents a deep curved sigmoid 

 groove, in which is lodged a part of the transverse sinus; the mastoid foramen is 

 seen opening into the groove. The interior of the mastoid portion, in the adult, 

 is usually occupied by cavities lined by mucous membrane and known as the 

 mastoid air-cells (fig. 97). These open into a small chamber — the mastoid 

 antrum — which communicates with the upper part of the tympanic cavity. The 

 mastoid cells are arranged in three groups: (1) antero-superior, (2) middle, and 

 (3) apical. The ^apical cells, situated at the apex of the mastoid process, are 

 small and usually contain marrow. 



Borders. — -The superior border is broad and rough for articulation with the hinder part of 

 the inferior border of the parietal bone. The posterior border, very uneven and serrated, 

 articulates with the inferior border of the occipital bone, extending from the lateral angle to 

 the jugular process. 



The petrous portion [pars petrosa; pyramis] is a pyramid of very dense bone 

 presenting for examination a base, an apex, three (or four) surfaces, and three (or 

 four) borders or angles. Two sides of the pyramid look into the cranial cavity, 

 the posterior into the posterior cranial fossa, and the anterior into the middle 

 cranial fossa. The inferior surface appears on the under surface of the cranium. 

 The medial and posterior walls of the tympanic cavity in the temporal bone 

 are sometimes described as a fourth side of the pyramid. The base forms a part 

 of the lateral surface of the cranium; the apex is placed medially. 



The posterior surface of the pyramid is triangular in form, bounded above by 

 the superior angle and below by the posterior angle Near the middle is an 

 obliquely directed foramen [porus acusticus internus] leading into a short canal — 

 the internal auditory meatus — at the bottom of which is a plate of bone, pierced 

 by numerous foramina, and known as the lamina cribrosa. The canal transmits 

 the facial and auditory nerves, the pars intermedia, and the internal auditory 

 artery. The bottom of the internal auditory meatus can be most advantageouslj' 

 studied in a temporal bone at about the time of birth, when the canal is shallow 

 and the openings relatively wide. 



The fundus of the meatus is divided by a transverse ridge of bone, the transverse crest, into a 

 superior and inferior fossa. Of these, the superior is the smaller, and presents anteriorly the 

 beginning of the facial canal (aqueduct of Fallopius), which transmits the seventh nerve. The 

 rest of the surface above the crest is dotted with small foramina (the superior vestibular area) 

 which transmit nerve-twigs to the recessus ellipticus (fovea heniielliptica) and the ampulla? 

 of the superior and lateral semicircular canals (vestibular division of the auditory nerve). 

 Below the crest there are two depressions and an opening. Of these, an anterior curled tract 

 (the spiral cribriform tract) with a central foramen (foramen centrale cochleare) marks the 

 base of the cochlea; the central foramen indicates the orifice of the canal of the modiolus, 

 and the smaller foramina transmit the cochlear twigs of the auditory nerve. The posterior 

 opening (foramen singulare) is for the nerve to the ampulla of the posterior semicircular 

 canal. The middle depression (inferior vestibular area) is dotted with minute foramina for 

 the nerve-twigs to the saccule, which is lodged in the recessus spha^ricus (fovea hemisphajri- 

 ca). The inferior fossa is subdivided by a low vertical crest. The fossa in front of the crest 

 is tho, fossulu cochlearis, and the recess behind it is the fossula vestibularis. 



Behind and lateral to the meatus is a narrow fissure, the aquaeductus vestibuli, covered by 

 a scale of bone. In the fissure lies the ductus endolymphaticus, a small arteriole and venule, 

 and a process of connective tissue which unites the dura mater to the sheath of the internal 

 ear. Occasionally a bristle can be passed through it into the vestibule. Near the upper 

 margin, and opposite a point about midway between the meatus and the aqueduct of the vesti- 



