Measurements of nasal resistance do not give an exhaustive functional assessment of nasal breathing function. Nevertheless, an increase in its thermal endocranial exchanges during its operation, which are regulated specifically by respiratory nasal mucosa, can be utilized to construct an operative physiological definition of nasal breathing at a state of optimal rest. These precise assessments, in elucidating the etiology of oral-nasal malformations, can give a clear picture of the morphological characteristics that orthodontists should strive to optimize as the face develops over the course of their treatment of malocclusions.