I don’t think as we went through symbolism that we ever touched on all the parts of the human body and what they mean. So let’s get to the foot of the matter and work our way up.
The foot is the hard worker of the body. It carries the weight of the rest of the body and is constant contact with the ground and the dirt, earthiness, and all things that fall to the ground (think of the injunctions to “cover one’s feet.) It comes in contact with filth so that the rest of the body does not have to. Conversely it may also be the reason that Moses was asked to remove his sandals before the burning bush. He is told that the ground and which he stood was holy – God, as it were, thought to be connected to a place, and by removing his sandals even his body now is in contact with that which is holy.
That being said, most of the time to be a foot is a very humbling thing, certainly not one of the more glorified parts of the body. So imagine the woman washing Jesus’ feet with her tears or Jesus washing the feet of his disciples, or John the Baptist saying that he is not worthy to loosen the strap of Jesus’ sandal; these are all acts of great humility and of great honor to the person to whom they are done.
In art when objects are placed under the feet of a person (picture the snake under Mary’s feet, or the dragon below Saint George, or a statue of the Resurrections with symbols of His Crucifixion under His feet) it means that they have conquered whatever those objects signify; they are even below this most humble part of this person for they have been completely set below them.