Subglottis: at short distance from the vocal cords a simple stenosis, in diaphragm, considerably reduces the light.
Ventilation is possible thanks to a central and circular hole, with a small marginal nodule.
The same volume of inhaled air must increase its speed to enter through a narrow light, becoming turbulent, which causes visible hyperemia in the mucosa.
Enlarged photograph of the case.
Note the thin and taut edges of the central diaphragm hole.
Almost always yield immediately to the dilation maneuvers.