This image's head was of fine gold
The prophet begins with the superior part of this image, and descends to the lower, because of the order and condition of the monarchies it represents: this signifies the Babylonian monarchy, as afterwards explained; called the "head", being the first and chief of the monarchies; and compared to "fine gold", because of the glory, excellency, and duration of it: his breast and his arms of silver;
its two arms, including its hands and its breast, to which they were joined, were of silver, a metal of less value than gold; designing the monarchy of the Medes and Persians, which are the two arms, and which centred in Cyrus, who was by his father a Persian, by his mother a Mede; and upon whom, after his uncle's death, the whole monarchy devolved: his belly and his thighs of brass;
a baser metal still; this points at the Macedonian or Grecian monarchy, set up by Alexander, signified by the "belly", for intemperance and luxury; as the two "thighs" denote his principal successors, the Selucidae and Lagidae, the Syrian and Egyptian kings; and these of brass, because of the sounding fame of them, as Jerom.