Sushumna extends from the Muladhara Chakra (second vertebra of coccygeal region) to Brahmarandhra. The Western Anatomy admits that there is a central canal in the Spinal Cord, called Canalis Centralis and that the cord is made up of grey and white brain-matter. Spinal Cord is dropped or suspended in the hollow of the spinal column.
In the same way, Sushumna is dropped within the spinal canal and has subtle sections. It is of red colour like Agni (fire). Within this Sushumna there is a Nadi by name Vajra which is lustrous as Surya (sun) with Rajasic qualities. Again within this Vajra Nadi, there is another Nadi, called Chitra. It is of Sattvic nature and of pale colour. The qualities of Agni, Surya and Chandra (fire, sun and moon) are the three aspects of Sabda Brahman. Here within this Chitra, there is a very fine minute canal (which is known as Canalis Centralis). This canal is known as Brahmanadi through which Kundalini, when awakened, passes from Muladhara to Sahasrara Chakra. In this centre exist all the six Chakras (lotuses, viz., Muladhara, Svadhishthana, Manipura, Anahata, Vishuddha and Ajna).
The lower extremity of the Chitra Nadi is called Brahmadvara, the door of Brahman, as Kundalini has to pass through this door to Brahmarandhra. This corresponds to Haridwar which is the gate of Hari of Badrinarayan in the macrocosm (physical plane). The Chitra terminates in the Cerebellum.
In a general sense the Sushumna Nadi itself (gross Spinal Cord) is called Brahma Nadi because, Brahma Nadi is within the Sushumna. Again the canal within the Chitra is also called Sushumna, because the canal is within the Sushumna. Ida and Pingala Nadis are on the left and right sides of the spine.
IDA AND PINGALA NADIS:
Ida and Pingala Nadis are not the gross sympathetic chains. These are the subtle Nadis that carry the Sukshma Prana. In the physical body these tentatively correspond to the right and left sympathetic chains.
Ida starts from the right testicle and Pingala from the left testicle. They meet with Sushumna Nadi at the Muladhara Chakra and make a knot there. This junction of three Nadis at the Muladhara Chakra is known as Mukta Triveni. Ganga, Yamuna and Sarasvati dwell in Pingala, Ida and Sushumna Nadis respectively. This meeting place is called Brahma Granthi. Again these meet at the Anahata and Ajna Chakra. In the macrocosm also you have a Triveni at Prayag where the three rivers Ganga, Yamuna and Sarasvati meet.
Ida flows through the left nostril and Pingala through the right nostril. Ida is also called Chandra Nadi (moon) and Pingala as Surya Nadi (sun). Ida is cooling and Pingala is heating. Pingala digests the food. Ida is of pale, Sakti Rupa. It is the great nourisher of the world. Pingala is of fiery red, Rudra Rupa.
Ida and Pingala indicate Kala (time) and Sushumna swallows time. The Yogi knows the time of his death; takes his Prana into Sushumna; keeps it in Brahmarandra, and defies time (Kala—death). The famous Yogi Sri Chang Dev of Maharashtra fought against death several times by taking the Prana into Sushumna. He was a contemporary of Sri Jnanadev of Alandi, near Poona. It was he who had Bhuta Siddhi, control over wild animals, through his Yogic practices. He came on the back of a tiger to see Sri Jnanadev.