ploxeni: explained by Festus to mean a wagon-box (capsum in cisio capsave), and said by Quintilian to be circumpadane (Gallic?) in origin; Quint. 1.5.8 “Catullus ‘ploxenum’ circa Padum invenit” . The comparison here may be of the wrinkled and fissured look of diseased gums to some peculiarity in shape of the ploxenum, or to its wrinkled and split rawhide covering.
 pistrino: etc., i. e. relegated to the occupation of the rudest slaves, that of driving the ass that turns the mill.