Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/27

Rh rated from the cerebral fossa by an incompletely ossified lateral partition, the tentorium. The ossified portion of the tentorium is a dorsal extension of the petrosal bone, which forms a part of the lateral wall of the cerebellar fossa. The floccular fossa is a relatively large cavity occupying the posterodorsal region of the petrosal bone. Its opening into the cranial cavity is an oval aperture considerably narrower than the cavity itself. The cerebral fossa is the largest division of the cranial cavity. It incloses the cerebrum. The foramina observed in the study of the lateral view may be identified easily by thrusting a bristle through each foramen from the outside. The olfactory fossa is a laterally compressed cavity opening behind into the cerebral fossa and bounded in front by the cribriform plate. This cavity contains the olfactory lobes of the brain. The branches of the olfactory nerve enter the nasal cavity through the numerous foramina in the cribriform plate. In a cranium from which the roof has been removed, the cribriform plate will be seen to be heart-shaped.

Though the nasal cavity can be satisfactorily studied in a longitudinal section of the skull, it can be seen to the best advantage if the dorsal and lateral bony wall of the cavity be removed. In doing this care should be taken to preserve intact the nasal bones and their delicate ventral processes, and while chipping away the lateral wall to injure as little as possible the delicate turbinal bones at the back and sides of the nasal cavity. The nasal cavity is tubular in form posteriorly (naso-pharyngeal duct), and terminates at the single posterior naris, or choana. Its external opening is through the two anterior nares at the anterior end of the snout. The nasal cavity is bounded dorsally by the nasal bones, posteriorly by the cribriform plate and maxillary bones, ventrally by the premaxillary,