Aker is an earth diety in the pantheon of the Egyptian gods. He presides over the junction of the eastern and western horizons in the Underworld. When the sun was to set over one horizon, it is believed that the sun travels through the "earth tunnel" to rise on the other end. Aker guards the entrance and exit of this "earth tunnel". Aker is also said to be one of the first Gods, older than even Geb Himself.
Aker's motif is depicted as both a single lion with two heads (with the sun disc between the two heads) or as two seperate lions with the sun disc between them as shown in many depictions. On occasion, Aker is depicted as having the foreparts of lions, but with the heads of humans. His motif also has certain alchemical properties, (and is on occasion) associated with alchemy.
Aker is also associated with the removal of poisons, and to cure someone who has swallowed an obnoxious fly. It is said that He also opens the earth gate for the king to pass through into the Underworld. Aker also imprisons the coils of the snake (The un-named one) after the un-named one is defeated by Aset.
The Book of Aker speaks of the solar journey from sunrise to sunset. While the book in it's entirety can no longer be found, it's components can be brought together for a complete reconstruction of the Book of Aker. These components were found in the tomb of Ramses VI, the tomb of Pedamenopet, the el-Asasif necropolis at Thebes, and among various mythological papyri of the priesthood of Amun (of the twenty first dynasty).
Not all sides of Aker are pleasant. This is the case when He is pluralized into the Akeru (earth-gods). The Akeru have been mentioned in the pyramid texts. There it is written that the peoples held the hope that they would elude the grasp of the Akeru.