Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/32

18 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; posteriorly 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 articulation of the lower jaw, lies ventral to the base of the zygomatic process.

The tympanic bulla contains the tympanic cavity, or cavity of the middle ear, which opens laterally through the external acoustic meatus.

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.

The interparietal is an ovoid bone of membranous origin inserted between the supraoccipital and the paired parietals. It meets the nuchal surface nearly at right angles.