Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/28

14 maxillary, and palatine bones. The two long prepalatine foramina lie in its floor, and it communicates by a foramen with each orbito-temporal fossa. A perpendicular median partition, the nasal septum, divides the nasal cavity into two nasal fossae. The septum is partly cartilaginous.

Each nasal fossa contains three very delicate, folded turbinal bones, which probably correspond to the maxillo-turbinal, nasoturbinal, and ethmoturbinal bones of other mammals. The first of these is an elongated thin sheet of bone attached ventrally to the inner surface of the pre-maxillary bone. The second is a process of the nasal bone. Its ventral edge turns outward and upward like the rolled edge of a stiff piece of paper. The ethmoturbinal bone is much more extensively convoluted than the other turbinals. The convolutions inclose spaces known as ethmoidal cells, whose long axes are, in general, directed antero-posteriorly. The ethmoturbinals are attached posteriorly to the cribriform plate. Examination of a skull in which the bone has been removed from the side of the nasal fossa shows that in the posterior part of the fossa the turbinal bones extend from the top to the bottom of the cavity and posteriorly almost to the optic foramen. The convolutions of the turbinal bones, together with the flesh covering them, are sometimes called the nasal labyrinth. In mammals the folds supported by the turbinals warm the air and extract foreign bodies from it. The epithelium covering the ethmoturbinals contains olfactory cells which are stimulated by odorous substances in the air.

Consult a text on comparative anatomy to determine the variations among vertebrates in the bones of the nasal cavity. Compare the nasal cavity of the rat with the same part in the turtle, frog, dogfish, etc. Assisted by a text on embryology outline the stages in the evolution of the nasal cavity.