It is said that there are 3 crores of nadi in the human body, of which some are gross and some are subtle. Nadi means a nerve or artery in the ordinary sense; but all the nadis of which the books on Yoga speak are not of this physical character, but are subtle channels of energy. Of these nadi, the principal are fourteen; and of these fourteen, ida, pingala, and sushumna are the chief; and, again, of these three sushumna is the greatest, and to it all others are subordinate. Sushumna is in the hollow of the meru in the cerebro-spinal axis. It extends from the Muladhara lotus, the Tattvik earth centre, to the cerebral region. Sushumna is in the form of Fire (vahni-svarupa), and has within it the vajrini-nadi in the form of the sun (surya-svarupa). Within the latter is the pale nectar-dropping chitra or chitrini-nadi, which is also called Brahma-nadi, in the form of the moon (chandra-svarupa,). Sushumna is thus triguna. The various lotuses in the different Chakra of the body (vide post) are all suspended from the chitra-nadi, the chakra being described as knots in the nadi, which is as thin as the thousandth part of a hair. Outside the meru and on each side of sushumna are the nadi ida and pingala. Ida is on the left side, and, coiling round sushumna, has its exit in the left nostril. Pingala is on the right, and, similarly coiling, enters the right nostril. The sushumna, interlacing ida and pingala and the ajna-chakra round which they pass, thus forms a representation of the caduceus of Mercury. Ida is of a pale colour, is moon-like (chandra-svarupa), and contains nectar. Pingala is red, and is sun-like (suryya-svarupa), containing venom, the fluid of mortality. These three rivers, which are united at the ajna-chakra, flow separately from that point, and for this reason the ajna-chakra is called mukta triveni. The muladhara is called Yukta (united)-tri-veni, since it is the meeting-place of the three nadi, which are also called Ganga (Ida), Yamuna (Pingala), and Sarasvati (sushumna), after the three sacred rivers of India. The opening at the end of the sushumna in the muladhara is called brahma-dvara, which is closed by the coils of the sleeping Devi Kundalini.