Nadi (yoga)

# Early Reference
An early version of the nadi system is mentioned in the Chandogya Upanishad, which says:

# Ida, Pingala and Sushumna
Amongst these ducts or nadis, three are of the utmost importance: the Medullar Shushumna, which interpenetrates the cerebrospinal axis from the perineum to the juncture of the lamboid and sagittal suture of the cranium, the lunar Serpentine Ida of the left side, of a pale color, negative polarity; the solar Serpentine Pingala of the right side, red color, positive polarity. Those are the main nadis, but in some tantric texts more than 72,000 nadis are cited . They all start from the central channel of the chakras to the periphery where they gradually become thinner.
The Sat-Cakra-Narupana, one of the earliest text on nadis and chakra, explicitally refer to these three main nadis, calling them Sasi, Mihira, Susumna.
In the space outside the Meru, 
placed on the left and the right, 
are the two Nadis, Sasi and Mihira. 
The Nadi Susumna, whose substance 
is the threefold Gunas, 
is in the middle. 
She is the form of Moon, Sun, and Fire; 
Her body, a string of blooming Dhatura flowers, 
extends from the middle of the Kanda to the Head, 
and the Vajra inside Her extends, 
shining, from the Medhra to the Head.

# Functions and Activities
Nadis are thought to carry a life force energy known as prana in Sanskrit, or qi in Chinese-based systems.  In particular prana (active) is supposed to circulate inside Pingala, while apana(passive) is supposed to circulate inside Ida. Inside Sushumna is supposed to circulate kundalini when awakened. 
The Ida and Pingala nadis are often seen as referring to the two hemispheres of the brain. Pingala is the extroverted(Active), solar nadi, and corresponds to the right hand side of the body and the left hand side of the brain. Ida is the introverted, lunar nadi, and corresponds to the left hand side of the body and the right hand side of the brain (crossing occurs in the optical chiasma). These nadis are also said to have an extrasensory function, playing a part in empathic and instinctive responses.
The two nadis are believed to be stimulated through different practices, including Pranayama, which involves alternate breathing through left and right nostrils, which would alternately stimulate respectively the left and right sides of the brain. The word nadi comes from the Sanskrit root nad meaning "channel", "stream", or "flow". The rhythmical breathing and special breathing techniques are supposed to influence the flow of these nadis or energetic currents. According to this kind of interpretation (which is the Yoga interpretation) the breathing techniques will purify and develop these two energetic currents and will lead to breathing special exercises whose goal is to awake kundalini.

# Western tradition and Interpretations
Sometimes the three main nadis (Ida, Pingala and Sushumna) are related to the Caduceus of Hermes:  "the two snakes of which symbolize the kundalini or serpent-fire which is presently to be set in motion along those channels, while the wings typify the power of conscious flight through higher planes which the development of that fire confers".. In this framework of mystic western esotericism welded with yoga concepts, sometimes the three nadis are related and named as alchemical sulphur and alchemical mercury 
"In the East, the symbol of the two serpents twisting on the rod corresponds to the two currents Pingala and Ida which coil around the Merudanda: the first is red, hot and dry, likened to the Sun and the Alchemic Sulphur; the second, Ida, is cold and wet, like the Alchemic Mercury and is correlated with the Moon for its silver pallor."