THE TEMPORAL BONES 43 



of communicating cells; below in the mastoid process they are 

 developed after puberty and are arranged vertically. Above 

 these is the antrum mastoideum, which is a horizontal cellular 

 cavity, a part of the middle ear; its roof and posterolateral 

 wall are formed from the petrous portion, and are continuous 

 with the roof and side of the tympanum. Its anteromedian 

 wall belongs to the mastoid. Below it connects with the mastoid 

 cells; its opening into the tympanum is large and on a level 

 with the foramen ovale, so the floor of the tympanum passes 

 in front into the Eustachian tube and behind into the mastoid 

 antrum. The superior border of the mastoid is rough, slopes 

 back, and articulates with the posteroinferior angle of the 

 parietal; the posterior border articulates with the occipital 

 between its lateral angle and jugular process. 



The pyramidal portion includes the petrous (stone) and 

 tympanic (drum) portions. 



The petrous portion is a four-sided pyramid with its base 

 turned out, and its long axis inward, forward, and slightly 

 downward. The axes of the two portions if prolonged would 

 meet at the posterior edge of the nasal septum. This portion 

 presents four borders superior, inferior, anterior, and posterior; 

 and four surfaces anterointernal, poster ointernal, anteroexternal, 

 and poster oexternal, also a base and an apex. The base is 

 concealed in its upper half by the squamous and mastoid, 

 and covered below, where these diverge, by the tympanic 

 portion. The apex is received into the angle between the great 

 wing of the sphenoid and the basilar process, and presents 

 the anterior orifice of the carotid canal, and forms the postero- 

 external boundary of the foramen lacerum. 



The anterointernal surface is in the middle fossa of the 

 skull, and separated from the squamous portion by the fissura 

 petrosquamosa. This surface presents a little behind its centre 

 the eminentia arcuata, covering the superior semicircular 

 canal; in front of this is a groove passing to the hiatus Fallopii, 

 which leads to the aqueduct of Fallopius; it transmits the 

 large superficial petrosal nerve and the petrosal branch of 

 the middle meningeal artery. Outside this are a groove and 

 small foramen for the small superficial petrosal nerve. Near 

 the apex the wall of the carotid canal is deficient; above this 

 is a shallow depression for the Gasserian ganglion. Between 

 the petrosquamous fissure externally and the hiatus Fallopii 



