Rigor Mortis happens about
3-6 hours after
death. It starts in the head, and goes down to the
feet. It begins to leave through the
feet and exits through the head. This has nothing to do with blood. That is post mortem lividty. Rigor mortis occurs as a result of a chemical change in the muscles of the decedent, causing them to become very stiff and difficult to manipulate.
Also, this has nothing to do with erections that is called a "priapism".