practice of tracing the sign of the cross on the forehead, not limited to the Roman province of Africa, where Tertullian lived, is shown by the fact that the Egyptian Origen, only some thirty years his junior, interprets similarly the passage of the Book of Ezekiel to which Tertullian referred, with the difference that Origen saw in the sign on the forehead of which Ezekiel spoke a reference not to the Greek letter tau, but to the tau of the Hebrew alphabet, which earlier had the shape of a cross. Origen took this to be a prophecy of the Early Christian custom of making a cross on their foreheads when undertaking some activity, especially prayer and reading of the sacred texts. Visual depictions Crucifixions The oldest extant depiction of the execution of Jesus in any medium seems to be the second-century or early third-century relief on a jasper gemstone meant for use as an amulet, which is now in the British Museum in London. It portrays a naked bearded man whose arms are tied at the wrists by short strips to the transom of a T-shaped cross. An inscription in Greek on the obverse contains an invocation of the redeeming crucified Christ. On the reverse a later inscription by a different hand combines magical formulae with Christian terms. The catalogue of a 2007 exhibition says: "The appearance of the Crucifixion on a gem of such an early date suggests that pictures of the subject (now lost) may have been widespread even in the late second or early third century, most likely in conventional Christian contexts". Better known is the Alexamenos graffito, discovered during excavation of the Paedagogium on Rome's Palatine Hill in 1857. Dated to the start of the third century, it portrays a humanoid with the head of a donkey (Christ or Anubis?) on a cross and includes an inscription in Greek. Another graffito, known as the Pozzuoli graffito, discovered in 1959, and dated by some to the first century, by others to the beginning of the second. shows someone (scholars disagree on whether it is a man or a woman) crucified with, at least as interpreted by M. Zaninotto, the wrists nailed to the crossbar. None of these ancient depictions is sufficiently detailed to give precise information on the details of how a crucifixion was carried out, but they all show the use of a cross with transom.