Dog nose bone anatomy - bbmpz.wohlfuehlen-petra.de The nasal cavity begins at the nostril, ends at the choanae, and is divided longitudinally by the nasal septum into two nasal fossae. The nasal planum is the pigmented, hairless, rostralmost surface of the external nose. The philtrum is the midsagittal external crease in the nasal planum. The nasal openings are referred to as nares or nostrils.