The single riderless horse that follows the caisson with boots reversed in the stirrups and an empty saddle symbolizes the departed will never ride again. It is said to date from the time of Genghis Khan, when the Mongols believed the spirit of a sacrificed horse went through a "gate to the sky" to serve its fallen warrior in the after-life.
This custom is just one of many protocols observed in a military funeral, or following the line of duty death of a police officer or firefighter. Sadly, Philadelphia has had plenty of practice in the past year, with the funeral of Officer John Pawlowski on Friday, we've now buried five officers who were killed in the line of duty in less than a year.