Śivasaṃhitā 5.180

The sixteen kalās in the cranial cavity are the complete lunar phases of soma: the integral cycle of nectar that condenses in the sahasrāra. This precise cosmological image maps the accumulation of amṛta at the apex of the cakra system as the full lunar disk, the point of maximum consciousness potency.

Dvir is twice, aṣṭa eight, kalā part or phase (especially lunar), yuta endowed or possessing. The quotation «that from which speech and mind turn back without reaching it» (yato vāco nivartante aprāpya manasā saha) is textual from Taittirīya Upaniṣad II.9, one of Vedanta’s most cited verses describing Brahman’s ineffability.

The integration of this Upaniṣadic verse into the tantric Śiva-saṃhitā marks its position at the confluence of India’s two great thought streams: the jñāna (gnosis) path of the Upaniṣads and the śakti path of tantrism. The final result is identical: a knowledge that cannot be verbalized because no mind produces it—it only receives it upon dissolving into its source.