THE BOXES OF THE SKULL 185 



arch is formed by a lateral and afterwards ventral projection, the 

 zygomatic process of the squamosal. The base of this process 

 bears ventrally the mandibular fossa, and dorsally, in association 

 with the body of the squamosal, the temporal fossa. The internal 

 surface of the squamosal forms a considerable portion of the wall 

 of the cranial cavity, the middle cranial cavity being, in fact, 

 broadest in this region. 



The Petrotympanic Bone 



The petrotympanic bone (os petrotympanicum) is a somewhat 

 oblong bone lying in the lateral wall of the cranium between the 

 posterior sphenoid and occipital bones. It is chiefly indicated 

 externally by the tympanic bulla and the bony external acoustic 

 meatus. It is articulated anteriorly with the alisphenoid and 

 squamosal, dorsally w4th the supraoccipital, and posteriorly with 

 the exoccipital. Except for the presence of the squamosal process 

 of the squamosal bone, the lateral and ventral surfaces are exposed 

 to the outside of the skull. The internal surface, with the ex- 

 ception of a small ventral portion which is articulated with the 

 basioccipital bone, is exposed to the posterior cranial fossa. 

 Only a small portion of the anterior surface is in contact with the 

 squamosal bone, the larger part being applied to the tentorium 

 cerebelli and forming with the latter a posterior wall for the middle 

 cranial fossa. The dorsal portion of the bone corresponds in thick- 

 ness with the wing of the supraoccipital with which it is articulated. 

 The posterior surface is applied to the anterior surface of the ex- 

 occipital, and is thus excluded both from the cranial cavity and 

 from the external surface of the skull. Viewing the skull from 

 behind, however, it is seen that a small dorsal portion protrudes 

 in a triangular space formed by the dorsolateral margin of the 

 exoccipital and the ventrolateral margin of the supraoccipital wing. 

 This portion is distinguishable by its pitted character. It forms 

 the mastoid portion (pars mastoidea) as distinguished from the 

 solid white petrous portion (pars- petrosa), which is exposed to 

 the cranial cavity, and which contains the structures of the internal 

 ear. The mastoid portion lies for the most part above the tympanic 

 cavity, but it is also continued ventrad between the external 

 acoustic meatus and the exoccipital as the mastoid process. The 



