From Hatha yoga pradipika with Hans Ulrich Rieker commentary -Quote 5

"As one opens the door with a key, so the yogi should open the gate to liberation
[moksha] with the kundalini. The great goddess [kundalini] steeps, closing with her mouth the
opening through which one can ascend to the brahmarandhra (crown of the head), to that place
where there is neither pain nor suffering. The kundalini sleeps above the kanda [where the nadis
converge]. She gives liberation to the yogi and bondage to the fool. He who knows kundalini knows
yoga. −−The kundalini, it is said, is coiled like a serpent. He who can induce her to move [upward]
is liberated. There is no doubt about it. −−Between Ganga and Yamuna sits a young widow,
arousing compassion. One should despoil her, for this leads to the supreme seat of Vishnu [her
spouse in sahasrara]. The sacred Ganga is ida [nodi] and Yamuna if pin gala [nodi]. Between ida and pingala sits the young widow kundalini.
You should awaken the sleeping serpent by grasping its tad. The shakti, when aroused, moves upward."


Once more remember the churning of the ocean of milk. The demons seized the head of the snake,
the gods took hold of the tail, and thus the work was accomplished.
Here we have the same process. The physically manifested powers, prana and apana, pull on the
head; that is where the current flows into the sushumna, which is closed by the head of the serpent.
The spiritual forces, however, work from the tail. We will presently learn about the nature of these
spiritual forces

Sandilya Upanishad quotes

This body is ninety-six digits in length.

Prana extends twelve digits beyond the body. He who through,

the practice of yoga reduces his prana within his body to make

it equal to or not less than the fire in it becomes the greatest

of

the yogins. In men, the region of fire which is triangular in

form and brilliant as the molten gold is situated in the middle

of the body

Lying in the middle of the navel and

above it, is the seat of kundalini. The kundalini sakti is of the

form of eight prakrtis (matter) and coils itself eight ways or

(times). The movement of vayus (vital airs) checks duly the

food and drink all round by the side of skandha(the web of life)

It closes by

its head (the opening of) the brahmarandhra, and during the

time of (the practice of) yoga is awakened by the fire (in the

apana); then it shines with great brilliancy in the akas of the

heart in the shape of wisdom.

Situated at

the back of the anus, it is attached to the spinal column and

extends to the brahmarandhra of the head and is invisible

and subtle and is vaishnavi (or has the sakti force of Vishnu).

On the left of Sushumna is situated Ida and on the right is

Pingala. The moon moves in Ida and the sun in Pingala.

The moon is of the nature of tamas and the sun of rajas.

The poison share is of the sun and the nectar of the moon.

They both direct (or indicate) time and Sushumna, is the enjoyer

(or consumer) of time.

By the

contraction of the muscles of the neck and by the contraction

of the one below (viz.,) Apana, the Prana (breath) goes into the

Sushumna which is in the middle from the west nadi.

Drawing up the Apana and forcing down the Prana from the throat,

the yogin free from old age becomes a youth of sixteen.

Having by contraction opened the door of kundalini, one

should force open the door of moksha. Closing with her mouth

the door through which one ought to go, the kundalini sleeps

spiral in form and coiled up like a serpent. He who causes this

kundalini to move he is an emancipated person. If this kundalini

were to sleep in the upper part of the neck of any yogin, it goes

towards his emancipation. (If it were to sleep) in the lower part

(of the body), it is for the bondage of the ignorant. Leaving the

two nadis, Ida and the other (Pingala), it (prana) should move

in the Sushumna. That is the supreme seat of Vishnu.

Samadhi is the union of the Jivatma (individual self) and

the Paramatma (higher self) without ,the threefold state, (viz.,

the knower, the known, and the knowledge). It is of the nature

of extreme bliss and pure consciousness.

The nadis or psychic passages

Source
*A Systematic Course in the Ancient Tantric Techniques of Yoga and Kriya  Swami Satyananda Saraswati

The nadis are pathways along which prana or

bioplasmic energy travels. In acupuncture

these pathways are known as meridians (at

least in modern western terminology). Various

ancient yogis said that there are seventy two

thousand nadis in the psychic body. Others

said the number is in the region of three

hundred and twenty thousand. Whatever the

number, we can definitely say that they are

numerous.

Of these nadis three are particularly important.

They are known as the ida, pingala and

sushumna. The most important one is the

sushumna, which runs within the spine2. It

should not be confused with or identified with

the physical spine that we are normally aware

of. This nadi runs from the region of the

perineum (between the anus and sexual organs)

to the region of the brain. It connects the

mooladhara chakra (chakra meaning pranic

or psychic centre) in the perineum to ajna

(mid-brain) chakra at the top of the spine

passing through various other chakras enroute.

Again these chakras should not be

regarded as physical entities; they are part of

the bioplasmic or pranic body. As such they

are located within but are not of the physical

body.

Emanating from the left side of the mooladhara

chakra and passing through the

intervening chakras in turn in a curving, crisscrossing

path, finally ending up on the left

side of the ajna chakra, is the ida nadi. Emerging

from the right side of the mooladhara chakra

and passing in a similar but opposite sense to

that of the ida nadi, up to the right side of ajna,

is the pingala nadi.

The ida and pingala nadis are the pathways

of the two different aspects of prana. They

represent the two opposite poles of the same

energy. The ida is denoted as being negative

and is also known as the chandra (moon) nadi.

The pingala, on the other hand, is positive and

is often called the surya (sun) nadi. Associated

with these two nadis are various attributes or

qualities.

These two nadis are associated with the two

nostrils: pingala with the right and ida with the

left. The ancient yogis, with their detailed

awareness of the human body, found that the

flow of breath did not pass equally through

each nostril. In fact at any given time it was

more than likely that the flow of air through

one nostril would be greater than the other

nostril. This is an aspect of our body that even

physiologists have never seriously researched.

No one seems to have asked the question:

"Why have we got two nostrils?" There should

be a good reason and perhaps it is directly

related to ida and pingala. Incidentally, the

reader can test this for himself taking care to

remove all mucus from the nose. Merely block

each of the nostrils in turn and you should

find that the flow is greater through one than

the other. If you don't find this to be so, then

try at a later time for occasionally the flows are

equal.

The yogis of antiquity discovered that when

the left nostril had the predominant flow of air

then the pranic flow in ida nadi was also

predominant. And when the right nostril had

the greatest flow the pingala had the greatest

flow of prana. The flow of air through the

nostrils and consequently the pranic flow

continually alternates. Under normal conditions

the flow through the left nostril is

predominant for about an hour and then the

right flow becomes predominant for about an

hour. During changeover from one to the

other, flows are equal and the prana flows

through the sushumna nadi. This generally

lasts for a few minutes. These flows can also be

artificially altered in response to individual

needs. The study of these flows, their meaning

and implications in life is the subject of the

science of swara yoga.

Shiva Samhita Chapter II, verses 13-23

13. In the body of man there are 3,500,000 nadis; of them the principal are fourteen;
14-15. Sushumna, Ida, Pingala, Gandhari, Hastijihvika, Kuhu, Saraswati, Pusa, Sankhini, Payaswani, Varuni,
Alumbusa, Vishwodari, and Yasaswani. Among these Ida, Pingala and Sushumna are the chief.
16. Among these three, sushumna alone is the highest and beloved of the Yogis. Other vessels are subordinate
to it in the body.
17. All these principal nadis (vessels) have their mouths downwards, and are like thin threads of lotus. They
are all supported by the vertebral column, and represent the sun, moon and fire.
18. The innermost of these three is chitra; it is my beloved. In that there is the subtlest of all hollows called

Brahmarandhra.
19. Brilliant with five colours, pure, moving in the middle of sushumna, this chitra is the vital part of body and
centre of sushumna.
20. This has been called in the Shastras the Heavenly Way; this is the giver of the joy of immortality; by
contemplating it, the greatYogi destroys all sins.
21. Two digits above the rectum and two digits below the linga (penis) is the adhara lotus, having a dimension
of four digits.
22. In the pericarp of the adhara lotus there is the triangular, beautiful yoni, hidden and kept secret in all the
Tantras.
23. In it is the supreme goddess Kundalini of the form of electricity, in a coil. It has three coils and a half (like
a serpent), and is in the mouth of sushumna.

Some verses from the Shiva Samhita II,1-5

1. In this body, the mount Meru – i.e., the vertebral column – is surrounded by seven islands; there are rivers,
seas, mountains, fields; and lords of the fields too.
2. There are in it seers and sages; all the stars and planets as well. There are sacred pilgrimages, shrines; and
presiding deities of the shrines.
3. The sun and moon, agents of creation and destruction, also move in it. Ether, air, water and earth are also
there.
4. All the beings that exist in the three worlds are also to be found in the body; surrounding the Meru they are
engaged in their respective functions.
5. (But ordinary men do not know it). He who knows all this is a Yogi; there is no doubt about it.