The nadi map describes thousands of channels through which prana moves, and singles out three. Ida runs on the left, is associated with the moon, with cooling, with the receptive and reflective; pingala runs on the right, is associated with the sun, with heat, with activity and outward drive. Sushumna runs through the centre, along the spinal axis, and is normally described as closed. The two side channels are said to cross at each chakra, which is where the caduceus resemblance comes from.
The claim about ida and pingala has an observable correlate. Nasal breathing is not symmetrical; one nostril is generally more open than the other, and the dominance alternates over a period of roughly an hour or two through the day. The yogic literature treats this as the visible sign of which channel is active, and swara yoga develops an entire practice around observing and deliberately switching it, matching activities to the channel currently dominant.
Sushumna is the one that matters. It is described as opening only when ida and pingala are balanced, which is the state the alternating dominance normally prevents, and this is precisely what nadi shodhana, alternate nostril breathing, is constructed to produce. When the two are equalised the central channel is said to open and kundalini can rise, which is why almost every serious pranayama sequence begins with balancing rather than with force. The channels themselves belong to the pranamaya layer described in the five sheaths.
Ida and pingala describe the oscillation you are already in; sushumna is what practice is for. To see the sheath these channels belong to, explore more of The Living Sanatan Timeline.
Part 85 of 100 — Series II: To the Eternal
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