The physical body has an anatomy visible to dissection — organs, vessels, nerves, bones. The yogic tradition describes a second anatomy that is not visible to dissection but is available to direct perception through sustained practice: the network of channels — Nadis — through which Prana flows.
The Goraksha Samhita puts the number at 72,000. The Shiva Samhita says 350,000. These are not literal counts but descriptions of the radical complexity of the Pranic network — the suggestion that the subtle body is at least as intricate as the physical body.
Of the many Nadis, three are primary.
Ida — the left channel, associated with the moon, the feminine principle, the left nostril, the parasympathetic nervous system, cooling and lunar qualities. Ida governs the introspective, receptive, integrating functions of the practitioner.
Pingala — the right channel, associated with the sun, the masculine principle, the right nostril, the sympathetic nervous system, heating and solar qualities. Pingala governs the active, expressive, projecting functions.
Ida and Pingala interweave around the central axis of the body, crossing at each of the major energy centres — the Chakras — in a pattern that corresponds remarkably closely to the caduceus symbol of Western medicine and to the double-helix structure of DNA.
Sushumna — the central channel, running through the centre of the spinal column from the base of the spine to the crown of the head — is the channel of liberation. When Ida and Pingala are balanced, Prana enters Sushumna. When Prana moves through Sushumna, meditation deepens spontaneously, the ordinary sense of separate selfhood relaxes, and the conditions for direct recognition of one's own nature are created.
Abhinavagupta's Tantraloka describes Sushumna as the royal road — Brahma Nadi — the channel that corresponds to Shiva himself in the architecture of the subtle body. The upward movement of Prana through Sushumna is the biological-energetic equivalent of the recognition of Shiva-nature: the same movement, at the level of Prana, that Pratyabhijna is at the level of consciousness. The path is dual — you can approach the recognition through the philosophical investigation of Pratyabhijna, or through the Pranic practices that open Sushumna, or ideally through both simultaneously, each supporting the other.
The most direct method for influencing the Nadi system in daily practice: Nadi Shodhana pranayama — alternate nostril breathing. By alternately closing the right and left nostrils, the practitioner directly influences the activity of Pingala and Ida. When both are breathing with roughly equal ease, Prana tends to enter Sushumna spontaneously. The quality of meditation that follows is noticeably different from the quality that precedes it.
The less obvious method: the quality of daily life. Every significant emotional experience, every chronic stress pattern, every habitual way of holding the body affects the Nadi system. The Nadis are not separate from the life — they are the life, expressed in its subtle dimension. The practice of living with intelligence — with the attention to Prana that the Tantric tradition prescribes — is itself Nadi cultivation.
The inner landscape has rivers. Learning to work with them is learning to work with the most intimate geography available to you.