Three concentric channels inside the sushumna, one within another, from the kanda to the crown.
Function
The later tantric anatomy of the central channel. Inside the sushumna runs the vajra nadi, inside the vajra the chitrini, and inside the chitrini the brahma nadi, the finest of the three, which is the passage kundalini takes to the crown and which the texts describe as a thread of lotus fibre or a streak of lightning. Each layer is finer than the one outside it, so the anatomy is a diagram of interiority rather than a set of tubes side by side.