Vocal chords? since in theatre its mostly acting
The correct answer would be the boundaries of cosmos. This can be seen in the religious idol called Nataraja or Shiva as Lord of Dance. This idol depicts Shiva as a Lord of Dance a cosmic dancer who is making preparations for god Brahma who will start the process of creation. In order to do that Shiva is performing his divine dance in order to cleanse the weary universe which would make way for the new one.
