Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/24

10 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 ventral 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 cribriform 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 incisor tooth. The root occupies a semicircular canal (alveolus) which extends back to the level of the infraorbital fissure. Compare the arrangement of the rat's teeth with 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 incisors. 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, maxillaries, and premaxillaries.