88 Anatomy of the Rabbit. 



which, in the typical mammalian skull, is filled by the lamina papyracea 

 of the ethmoid. Its internal surface is associated with the ethmoid bone 

 and with the nasal cavity. It falls for the most part below the level 

 of the cranial cavity. 



4. THE SQUAMOSAL BONE. 



The temporal bone, or temporal complex, as recognized from the 

 human condition, is an association of three elements — squamosal, 

 tympanic, and periotic — which in the human skull are coalesced to form 

 a single bone. It is usually described as consisting of four portions, 

 of which the squamosal and tympanic portions are two, while the periotic 

 bone is considered to consist of two others, one of which, the petrous 

 portion, is a solid white portion lodging the internal ear, while the 

 second, or mastoid portion, is a mass of less compact character appear- 

 ing externally in the wall of the skull. In the rabbit the original 

 elements are not coalesced, but the periotic and tympanic bones are 

 so closelv associated that it is proper to describe them as forming a 

 petrotympanic 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 supraoccipital and petrotympanic, and 

 ventrally with the alisphenoid. Its posterior margin bears a prominent, 

 slightlv decurved squamosal 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 somewhat oblong 

 bone Iving 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 supra- 

 occipital, 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 



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