Suṣumnā
Definition: f. a particular artery (prob."the carotid") or vein of the body (lying between those called iḍā - and piṅgalā - ,and supposed to be one of the passages for the breath or spirit;
The suṣumnā-nāḍī (a tube-like structure meant for the passage of the kuṇḍalini) is situated in the middle of the body, stretching from the mulādhāra plexus at the root of the spinal column, up to the brahmarandhra or the aperture in the crown of the head.
It shines like the sun or the moon. The Kuṇdalinī which is fine like the thread in the lotus stalk, is inside this suṣumnā and is shining like millions of lightnings.
By closing the ears and meditating on the sound within and the blue light between the eyes, and witnessing it, one gets infinite happiness.
This is meditation from ‘inside.’
The Upaniṣad then proceeds to describe the process of meditation on two points.
In both the cases this light becomes a symbol for Brahman.
Then comes the description of ‘tārakayoga,’ meditation on the light in the head, by reversing the mental light into the area between the eye brows, inside the head but towards the crown. Such meditation can give the supernatural ‘aṣṭasiddhis’ or ‘eight powers.’
