The Bones of the Skull. 145 



in the wall of the skull. In the rabbit the original elements are not 

 coalesced, but the periotic and tympanic bones are so closely 

 associated that it is proper to describe them as forming a petro- 

 tympanic bone. 



The squamosal bone (os squamosum) is a rectangular plate, 

 forming part of the lateral wall of the cranium, and bearing the 

 posterior root of the zygomatic arch. It is articulated anteriorly 

 with the orbitosphenoid and with the orbital portion of the frontal, 

 dorsally with the frontal and parietal, posteriorly with the supra- 

 occipital and petrotympanic, and ventrally with the alisphenoid. 

 Its posterior margin bears a prominent, slightly decurved squa- 

 mosal process (processus squamosus). It lies on the lateral 

 surface of the petrotympanic immediately above the opening of the 

 bony external acoustic meatus. The posterior root of the zygo- 

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

 the zygomatic process of the squamosal. Its base bears ventrally 

 the mandibular fossa, and dorsally, in association with the body 

 of the squamosal, the temporal fossa. The int£rnal 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. 



5. THE PETROTYMPANIC BONE. 



The petrotympanic bone (os petrotympanicum) is a some- 

 what 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 with 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 is exposed to the 

 posterior cranial fossa, with the exception, however, of a small 

 ventral portion which is articulated with the basioccipital bone. 

 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 



