Humidification of the air passages.

The reasons for moistening of the mucous membranes are several. Ciliary action is dependent on a supply ofmucus, of the right viscosity and in correct amount; in this blanket of mucus are carried lysozymes, described by Fleming (1928), of considerable bacteriostatic and bactericidal importance. The alveoli of the lung must be kept moist, for otherwise respiratory exchanges cannot be carried on; the origin of this necessity is to be found in fish which left the sea to pass part of their lives on land, as for example the climbing perch. The olfactory area must have a supply of moisture to dissolve the particles which convey the sensation of smell; the vocal folds must be lubricated with mucus, for otherwise they will not vibrate. The pharynx must not be dry if deglutition is to be efficient; here saliva helps, but not continuously. Transudation helps to some degree in the respiratory tract, but not by itself, since evaporation would be excessive if the inspired air were charged with insufficient moisture; there would be consequent damage to the mucosa and deleterious effects, to be considered later.