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".