Menthol is an extract of the peppermint plant and is also known as peppermint oil.

Menthol acts as a counter irritant, local stimulant, and used in combination with camphor acts to relieve muscle aches and pain.

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

http://www.herbalremedies.com/mint-information.html

http://www.swedish.org/110998.cfm