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 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. 
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 
