8 ANATOMY OF THE RAT 



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 

 lamhdoidal 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 orhito-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 fis- 

 sure. 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: 



