The phrase "duct wall" refers to the barrier or surface that separates different areas within a duct. It is essentially the wall or boundary that helps to define the pathways within a duct system. Full definition
Enter outside air duct pressure (carefully make hole for pressure probe through duct wall; see Addendum II for a table relating duct pressure to airflow) (buildingscience.com)
A ductogram, in which a catheter is inserted into a lactation duct through the nipple to either inject radioactive dye that can be detected on x-ray or insert a miniature camera to visualize the internal duct walls, does not affect milk production or safety since there are no incisions or tissue removal and the radioactive dye is not absorbed into either the mother's or the baby's tissue (see below). (breastfeedingtoday-llli.org)
Biliary cirrhosis is the rarest form, caused by scarring of the bile duct walls and the surrounding liver tissue thought to represent an end stage of chronic biliary tract disease. (texaswestveterinarian.com)