Detail of the wall of the glandular stomach of a chicken (HE)
The mucosa of the proventriculus is projected into fingerlike folds (plicae) of variable length, separated from each other by intervening depressions (sulci). The l. propria in and just underneath the plicae is infiltrated with numerous lymphocytes which can be organized into lymphatic nodules. The plicae encircle a conical protuberance, the papilla, through which the primary duct opens into the lumen. In this picture we can see how a secondary duct passes onto a primary duct.