Vijnana Bhairava Tantra · Dharana 12 · Verse 35
Inner space of the central channel
Sense the center line of the body as a thin, hollow channel running from the base to the crown, as fine as the fiber of a lotus stem. Do not add pictures to it. Rest in the empty interior of that channel, not on its walls. You are attending to a space that happens to run through the middle of the body.
Older line
The central channel stands in the middle of the body, slender as the fiber of a lotus stem. Meditate on the space within it, and through that goddess of inner space the Divine shines forth.