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Nāḍi (Sanskrit नाडी nāḍī = tube, pipe; Tamil i//naadi) நாடி = nerve, blood vessel, pulse) are a term for the channels through which, in traditional Indian medicine and spiritual science, the energies of the physical body, the subtle body and the causal body are said to flow. Within this philosophical framework, the nadis are said to connect at special points of intensity called nadichakras. The concepts of a subtle body and a causal body are not recognised terms used in conventional science or medicine.
The word "nadi" is pronounced as "nāḍī", with long vowel sounds and a retroflex 'd'. In normal biological reference, a nadi can be translated as "nerve", "vein", or "artery" in English. The literal meaning is a long tubular organ which one could extend to include lymphatic channels and other biological systems (compare with kosha, a bag like organ). However, in yogic, and specifically in Kundalini yoga reference, a nadi can be thought of as a channel (not an anatomical structure). In regard to Kundalini yoga, there are three important nadis: ida, pingala, and sushumna. Ida (इडा, iḍā) lies to the left of the spine, whereas pingala (पिङ्गल, piṅgala) is to the right side of the spine, mirroring the ida. Sushumna (सुषुम्णा, suṣumṇā) runs along the spinal cord in the center, through the seven chakras – Mooladhaar at the base, and Sahasrar at the top (or crown) of the head. It is at the base of this sushumna where the Kundalini lies coiled in three and a half coils, in a dormant or sleeping state.
Several of the ancient Upanishad texts use the concept of nadis (channels). An early version of the nadi system is mentioned in the Chandogya Upanishad, which is believed to be about 3,000 years old, for example, comparing and connecting them with the rays of the Sun. When discussing what happens after death for the enlightened, Chandogya Upanishad states:
"A hundred and one are the arteries of the heart, one of them pierces the crown of the head. Going upward through that, one becomes immortal at death. Other arteries, going in different directions, only serve as channels for his departure from the body, ..." (CU 8.6.6)
"The nadis penetrate the body from the soles of the feet to the crown of the head. In them is prana, the breath of life and in that life abides Ātman, which is the abode of Shakti, creatrix of the animate and inanimate worlds." (VU 54/5)
Functions and activities
In the yoga theory, nadis are said to carry life force energies known as prana in Sanskrit. In the physical body, the nadis are channels carrying air, water, nutrients, blood and other bodily fluids around and are similar to the arteries, veins, capillaries, bronchioles, nerves, lymph canals and so on. In yoga theory, the physical body is often referred to as "the gross body" (Sanskrit Sthula sarira) in English.
In the subtle and the causal body, the nadis are channels for so called cosmic, vital, seminal, mental, intellectual, etc. energies (collectively described as prana) and are important for sensations, consciousness and the spiritual aura.
Many nadis are named after their function and the Shiva Samhita mentions a total of 350,000 nadis in the human body. The Samhita further explains, that 14 of these nadis are particularly important and that the three most vital of these are the Sushumna, Pingala and the Ida nadis.
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 (there is a contralateralization). 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 Pranayama practices, including nadi shodhana, 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 among seven Nadi Chakras in our body. Of the seven, four are in the trunk of the body, two in the head- and there is one in the neck. Chakra is the power center associated with the subtle body of man. Each chakra is ruled by an incarnation of Goddess Parashakti.
Ida, Pingala and Sushumna
Amongst these ducts or nadis, three are of the utmost importance: the Medullar Sushumna, which interpenetrates the cerebrospinal axis from the perineum to the juncture of the lamboid and sagittal suture of the cranium, and it is associated with both nostrils being open and free to the passage of air. The 'lunar Serpentine Ida' of the left side, of a pale color, negative polarity. It is associated with feminine attributes, the moon, the Yin element of Chinese philosophy, and an open left nostril. The solar Serpentine Pingala of the right side, red color, positive polarity. It is associated with masculine attributes, the light of the sun, the Yang element of Chinese philosophy, and an open right nostril. 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-Nirupana, one of the earliest texts on nadis and chakra, explicitly refer to these three main nadis, calling them Sasi, Mihira, and Susumna.
In the space outside the Meru, the right apart from the body 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 even water also; 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. 
Sushumna (alternatively known as Susumna) Nadi connects the base chakra to the crown chakra. It is very important in Yoga and Tantra in general. Alternative medicine also refers to Sushumna sometimes. In Raja Yoga or Yoga of Patanjali, when the mind is quietened through Yama, Niyama, Asana and Pranayama the important state of Pratyahara begins. A person entering this state never complains of Dispersion of Mind. This is characterised by observing the movements/jerks in Sushumna, the central canal in the subtle body. The movements indicate the flow of Prana through the central canal and in the process, the sushumna makes the way for the ascent of Kundalini.
Pingala is associated with solar energy. The word pingala means "tawny" in Sanskrit. Pingala has a sunlike nature and male energy. Its temperature is heating and courses from the right testicle to the right nostril. It corresponds to the river Yamuna.
Ida is associated with lunar energy. The word ida means "comfort" in Sanskrit. Idā has a moonlike nature and female energy with a cooling effect. It courses from the left testicle to the left nostril and corresponds to the Ganges river.
The Ida and Pingala nadis are often seen as referring to the two hemispheres of the brain. Pingala is the extroverted, solar nadi, and corresponds to left hemisphere . Ida is the introverted, lunar nadi, and refers to the right hemisphere of the brain. Ida nadi controls all the mental processes while Pingala nadi controls all the vital processes.
Other traditions and interpretations
Other cultures of the world are also working with similar concepts of the nadis and prana energies.
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.
- "Light on Pranayama" (Ch. 5: Nadis and Chakras)
- For reference to Chandogya Upanishad 8.6.6 and interpretation as an early form of the occult physiology see: McEvilley, Thomas. "The Spinal Serpent", in: Harper and Brown, p.94.
- Varahopanisad V, 54/5.
- Other texts explains there are 72,000 nadis in the human body and each branches out to another 72,000 nadis.
- Arthur Avalon, The Serpentine Power (collection of yoga-tantric texts)
- Tommaso Palamidessi, The Occult Constitution of Man and Woman, Bio-energy and the energetic ducts
- Image of 72.000 nadis
- Sat-Cakra-Narupana, The Muladhara Cakra, transl. Sir John Woodroffe in The Serpent Power: Being the Ṣaṭ-cakra-nirūpana and Pādukā-pañcaka
- C. W. Leadbeater, Chakras, Adyar, 1929
- J. Evola, La Tradizione Ermetica, 1932
- T.Palamidessi, Alchimia come via allo Spirito, ed. EGO, 1948
- Tommaso Palamidessi
- Katha Upanishad, probably 1,000 BCE
- Varaha Upanishad
- Sat-Cakra-Nirupana, Purnananda Swami
- Shiva Samhita, probably the 1600s
- B. K. S. Iyengar (2010). Light on Pranayama. the Crossroad Publishing Company.