If P.Karthigayan in his "History of Medical and Spiritual Sciences of Siddhas of Tamil Nadu", Notion Press, 2016, explains that the soul is the bridge between the spirit and the body,
The body is composed of physical and spiritual matters sandwiched by a bridging factor known as soul. Body is a functional machine driven by spirit operated through the components in the bridging network of soul. (https://agathiyarvanam.blogspot.com/2018/10/spirit-soul-various-bodies.html)
BKS Iyengar in his "Light on the Yoga Sutras of Patanjali", HarparsCollins Publishers India, 1993, explains the difference between the soul and the individual self. He gives a simple metaphor. If the soul is the granite statue of Erai that sits in the inner sanctum of the temple, the self is the bronze statue that is taken around the temple grounds as a representative of the permanent statue.
If the unifying factor between the spirit and the body is the soul, the unifying factor between the soul and the body are the layers or sheaths namely beginning with the physical body itself, the Annamaya kosam, next the physiological sheath or Pranamaya kosam, traveling further in we meet the mental sheath or Manomaya kosam, and going further within we meet the intellectual sheath or Vijnanamaya kosam before finally encountering the pure blissful sheath or Anadhamaya kosam. BKS Iyengar says these sheaths represent the five elements of nature: earth, water, fire, air and ether. He throws in another sheath, the Mahat or cosmic consciousness in its individual form as citta; and the inner soul as the seventh sheath.
The anatomical sheath or the gross body is known as the kariya sariram; the physiological, mental and intellectual sheaths form the subtle body or suksma sariram. The blissful sheath is called the causal body or karana sariram.
Everything is in a state of expansion. Science says that the universe is growing and expanding. Mark A Garlick writes in his "The Expanding Universe", Dorling Kindersley, London 2002, "Astronomers know that the Universe is expanding and that the expansion is speeding up."
He gives three possibilities to the end of the universe: "Either the universe will go on expanding forever; or it will one day reverse and start to collapse; or it will oscillate between expansion and contraction forever."
Myths too says Erai grows as told in the story of Brahma and Vishnu vying to reach the crown and feet of Shiva respectively.
The physical body grows when fed. The soul too expands according to P.Karthigayan, gaining new experience each day. The spark of spirit is fanned into a majestic flame with daily devotion and through the show of love as revealed by the mystics and Siddhas. Nothing seizes to stop growing or changing.
Pandit Gopi Krishna in his book "Kundalini - Path to Higher Consciousness", Orient Paperbacks, 1976, speaks of "the existence of a subtle life element prana."
"The existence of a subtle life element prana, the cosmic life energy and its subtle biological conductor in the body which pervades each cell of every tissue and fluid, that sustains the activity of the brain and nervous system. The prana has a biological component in the brain, an extremely fine biochemical essence of a highly delicate and volatile nature extracted by the nerves, generating a subtle radiation."
"The existence of a subtle life element prana, the cosmic life energy and its subtle biological conductor in the body which pervades each cell of every tissue and fluid, that sustains the activity of the brain and nervous system. The prana has a biological component in the brain, an extremely fine biochemical essence of a highly delicate and volatile nature extracted by the nerves, generating a subtle radiation."
Studying the yoga systems he says that "living bodies owe their existence to prana, an extremely subtle immaterial substance pervading the universe which is the cause of all organic phenomena controlling the organisms by means of the nervous system and the brain, manifesting itself as the vital energy."
He adds that "normally the work of extraction of prana to feed the brain is done by a limited group of nerves, but with the awakening of Kundalini, other and more extensive groups of nerves are stirred to activity, leading to the transmission of an enormous and enhanced supply of a more concentrated form of pranic radiation into the brain drawn, from a vastly increased area of the body."
Pandit Gopi Krishna who "had aroused this physic energy in his life" speaks about the result of that sudden outburst one day after putting in years of meditation, his first encounter with the serpent energy where "he had his first glimpse into the super conscious state and saw fabulous Kundalini in action." He felt himself slipping out of the body.
I felt the point of consciousness that was myself growing wider and wider, spreading outward while the body, appeared to have receded into the distance. I became all consciousness.
I was no longer myself or to be more accurate no longer as I knew myself to be a small point of awareness confined in a body, but instead was a vast circle of consciousness in which the body was but a point.
He explains,
There had been an expansion of his own self, my own consciousness and the transformation had been brought about by the vital current that had started from below the spine and found access to my brain through the backbone.
He reminds us that raising the Kundalini is not for the faint-hearted.
Considering the colossal nature of the physical and mental metamorphosis that has to be effected as a prelude to spiritual unfoldment, I do not wonder at the accompanying trials and tribulations since the mystic state represents the last and most arduous lap of the journey which began with mans ascent from dust, it terminates with his tasting, after suffering and travail the incomparable bliss of unembodied existance, not after death but within his span of life on earth.
He states that the ancients considered Kundalini as "the queen of the nervous system, controlling all the thousand of nadis or nerves in the body and for the same reason have designated her as Adhara Shakti on which depends the existence of the body and the universe, microcosm and macrocosm."
The Pandit clarifies that oxygen is not prana but rather a vehicle for prana to travel and transverse throughout the body.
The air we breathe is permeated with prana and this vital current flows through us along with the air we breath. Breath and prana are not identical. Prana utilises oxygen as the main vehicle for its activity.
Prana is an inseparable part of the cosmic energy or Shakti. Shakti when applied to inorganic matter is force and when to the organic plane, is life. In yoga parlance prana is life and life is prana. Life and vitality in the sense used here do not mean soul or the spark of the divine in man. Prana is merely the life energy by which divinity brings into existence the organic kingdom and acts on the organic structure as it creates and acts on the universe by means of physical energy.
He says the earth has its own supply of prana, pervading every atom and molecule. Similarly the sun constantly sends out "an enormous supply of pranic radiation on earth." The moon too is a big supplier of prana for earth, and so are all the other planets and stars, all inexhaustible stores of prana.
Walking bare foot and grounding oneself with Mother Earth, gives abundance prana. Moving into the sunshine provides additional prana. Taking long dips in rivers, lakes, seas and other sources of water, even in our bathtubs and during showers, increases prana. Rapidly inhaling and exhaling brings more prana within that can be felt expanding to the verge of the body bursting and rupturing. Sitting in front of an open fire provides more prana. A walk in the open through the woods or green meadows, energizes us thanks to the prana.
Walking bare foot and grounding oneself with Mother Earth, gives abundance prana. Moving into the sunshine provides additional prana. Taking long dips in rivers, lakes, seas and other sources of water, even in our bathtubs and during showers, increases prana. Rapidly inhaling and exhaling brings more prana within that can be felt expanding to the verge of the body bursting and rupturing. Sitting in front of an open fire provides more prana. A walk in the open through the woods or green meadows, energizes us thanks to the prana.
Similarly with continuous devotion the spirit expands to fill the entire body engulfing in flame of love.