 In the arms of the cross are set two lead caskets, one containing a fragment of the True Cross and a relic of St. Andrew and the other containing medallions of the Holy Lamb.In the mid-18th century, cracks appeared in the dome, so four iron chains were installed between the two shells to bind it, like the rings that keep a barrel from bursting.As many as ten chains have been installed at various times, the earliest possibly planned by Michelangelo himself as a precaution, as Brunelleschi did at Florence Cathedral.