10 ANATOMY OF THE RAT 



ovale is the large anterior lacerated foramen. The optic 

 foramen for the passage of the second cranial nerve is 

 anterodorsal to the anterior lacerated foramen. 



There is a pronounced groove along the ventral side of 

 the orbito-temporal fossa. The small postpalatine foramen 

 communicates near the center of this groove with the ven- 

 tral surface of the palatine bone. Anterior to this foramen 

 another foramen establishes a connection between the orbit 

 and the nasal cavity. A small foramen dorsal to these 

 openings perforates the cranium just posterior to the crib- 

 riform plate, which is described later. 



The maxillary bone is incompletely ossified medial to 

 the infraorbital fissure. Consequently in cleaning the skull 

 the nasal cavity is usually opened at this place. 



The convex lateral surface of the premaxillary bone 

 reveals the course of the root of the enormous curved in- 

 cisor tooth. The root occupies a semicircular canal (alve- 

 olus) which extends back to the level of the infraorbital 

 fissure. Compare the arrangement of the rat's teeth vsdth 

 that of the cat, dog, and man, for instance. The rat has 

 no canine teeth, which are so prominent in carnivores, and 

 no premolars. The three molar teeth are inserted in the 

 maxillary bone a considerable distance posterior to the in- 

 cisors. The toothless space between the incisor and molar 

 teeth is the diastema. A similar arrangement occurs in 

 the horse, for instance. 



Exercise II. Draw and label the lateral view of the cranium. 



Ventral view of the skull. The following bones are seen 

 from this view: the occipital, which is flanked on either 

 side by the petrosal bone and the tympanic bulla, the 

 basisphenoid with an alisphenoid fused to each side, the 

 presphenoid and the paired squamosals, palatines, maxil- 

 laries, and premaxillaries. 



