What tissue of the body is in rigor mortis?

What is the physiological explanation for rigor mortis?

What is the usefulness of the phenomenon of rigor mortis in forensic medicine?

Assume the body is buried in a casket and exhumed five years later for a medical test. If the biceps brachii muscle was examined, what might you expect to find and why?

Primarily the muscles... although the skin stiffens some too... :
http://www.deathonline.net/decomposition/body_changes/rigor_mortis.htm

http://en.wikipedia.org/wiki/Rigor_mortis