Is a multi-tissue structure part of the maxillary barbel blood sinus which is connected to the central chamber of the maxillary blood sinus by a slight isthmus. It is not muscularized, has no dense connective tissue capsule and is not attached to adjacent bone. Is composed of dark tunica intima wrapped by thick, light tunica adventitia; cells within the adventitia are widely separated, with extended cytoplasmic veils. External to the adventitia, a branch of the facial nerve and several smaller nerve bundles appear in cross section just lateral to the isthmus. These nerves are in the dermis, but outside the sinus. [ http://zfin.org/ZDB-PUB-140813-6 GOC:ymb ORCiD:0000-0002-9900-7880 ]