Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/22

8 surface of the rat's skull are the sutures where the adjacent bones are joined to one another.

The extreme posterior, or nuchal, surface of the skull is nearly perpendicular to the long axis of the animal, and joins the dorsal surface almost at a right angle. The lambdoidal ridge occupies the region of union of these two surfaces. It is semicircular in form. Anterior to the lambdoidal ridge is a second ridge extending ventrally to the region of the ear. Dorsally this second ridge joins the temporal line at right angles. This line extends forward laterally on the dorsal surface of the cranium to the orbit, where it turns medially and disappears. Compare the rat with the horse and cat, for instance, where the lambdoidal ridge is well developed, and with the human skull where it is absent.

The orbito-temporal fossa is a deep depression on each side of the mid-region of the cranium, about one-third as long as the whole cranium. It contains the eye, eye muscles, and muscles, which help close the jaw. The zygomatic arch bounds it laterally. This arch is a bow-shaped bar of bone which extends forward from the articulation of the lower jaw. At its anterior end this arch broadens and is pierced by the large infraorbital fissure. Viewed from the anterior end this fissure is shaped somewhat like a palm-leaf fan, broad dorsally and narrow ventrally. In other vertebrates the orbito-temporal fossa is more or less completely separated by bony processes into two distinct cavities, the orbit for the eye and its muscles, and the temporal fossa.

The anterior nares [piriform aperture in man] are located at the anterior extremity of the cranium. Air enters through these openings and is conveyed through the nasal passages to the lungs.

The following bones may be seen from the dorsal side: