The round vessel or stronghold is the skull. “The divine organ,” says the “Liber quartorum,” “is the head, for it is the abode of the divine part, namely the soul.” That is why the Philosopher must “surround this organ with greater care than other organs.” Because of its roundness, “it attracts the firmament and is by it attracted; and it is attracted in similar manner by the attracter, until the attraction reaches its end in the understanding”