18 ANATOMY OF THE RAT 



ventrally by the petrous and tympanic bones, and by an 

 unossified region of the skull. The ''claw" of the hammer 

 is bounded ventroanteriorly by the alisphenoid bone; pos- 

 teriorly its tip approaches the tympanic bone. Dorsally 

 the "hammer" joins the frontal and parietal bones; it is 

 bounded anteriorly by the orbital process of the frontal 

 bone. 



The squamosal bone bears a pronounced longitudinal 

 ridge which runs nearly its whole length. Dorsal to the 

 **claw" the ridge is extended anteroventrally as a curved 

 spur, the zygomatic process. This process forms a part 

 of the zygomatic arch. The mandibular fossa, for the ar- 

 ticulation of the lower jaw, lies ventral to the base of 

 the zygomatic process. 



Tympanic Bone (Os Tympanicum) 



The tympanic hvZla contains the tympanic cavity, or 

 cavity of the middle ear, which opens laterally through 

 the external acoustic meatus. 



Petrosal Bone (Os Petrosum) 



The petrosal bone incloses the inner ear, and is seen 

 on the inner surface of the skull. The mastoid portion 

 of the petrosal is inserted into the notch at the side of 

 the occipital bone. 



Interparietal Bone (Os Interparietale) 



The interparietal is an ovoid bone of membranous ori- 

 gin inserted between the supraoccipital and the paired 

 parietals. It meets the nuchal surface nearly at right 

 angles. 



