It’s the time to expand the topic of “internal fire”. Accidentally, I discovered a very interesting description of the inner nāda source, which occurs in Bṛhadāraṇyaka Upaniṣad, one of the earliest Upaniṣadas. The śloka from there is also repeated in Maitri (Maitrāyaṇīya) Upaniṣad (2.6.).
बृहदारण्यक उपनिषद् । ५.११.९ ।
bṛhadāraṇyaka upaniṣad । 5.11.9 ।
अयमग्निर्वैश्वानरो योऽयमन्तः पुरुषे येनेदमन्नं पच्यते यदिदमद्यते तस्यैष
घोषो भवति यमेतत्कर्णावपिधाय शृणोति स यदोत्क्रमिष्यन्भवति नैनं घोषं शृणोति ॥
ayamagnirvaiśvānaro yo’yamantaḥ puruṣe yenedamannaṃ pacyate
yadidamadyate tasyaiṣa ghoṣo bhavati yametatkarṇāvapidhāya śṛṇoti sa
yadotkramiṣyanbhavati nainaṃ ghoṣaṃ śṛṇoti ॥
The fire, in the form of a universal being, is inside the living beings, thanks to it the food that is eaten is digested. He [a man], hears exactly that noise when he closes his ears. [However] he does not hear this sound when he dies.
Vaiśvānara is a very revered deity in the Vedas, much like Agni, and is often identified with him. It is also one of the deities that is recommended to perform dhyāna on in Yogayājñavalkya and Vasiṣṭha Saṃhitā. We can find many parallels with various yoga sources. For example, the following signs associated with purified nāḍis are listed in Haṭha Yoga Pradīpikā:
यदा तु नाडीशुद्धिः स्यात्तथा चिह्नानि बाह्यतः।
कायस्य कृशता कान्तिस्तदा जायेत निश्चितम् ॥१९॥
yadā tu nāḍīśuddhiḥ syāttathā cihnāni bāhyataḥ।
kāyasya kṛśatā kāntistadā jāyeta niścitam॥19॥
When the nāḍis are purified, then external signs appear: harmony and radiance of the body, the yogi has success beyond doubt.
यथेष्टं धारणं वायोरनलस्य प्रदीपनम्।
नादाभिव्यक्तिरारोग्यं जायते नाडिशोधनात् ॥२०॥
yatheṣṭaṁ dhāraṇaṁ vāyoranalasya pradīpanam ।
nādābhivyaktirārogyaṁ jāyate nāḍiśodhanāt ॥ 20 ॥
If the yogi can retain the life force (e.g. kumbhaka), then the radiance and inner fire are increasing. Having cleaned channels, the nāda sounds are heard and disease is overcome.
Of course, there are different types of agni in the body among which jaṭharāgni is considered particularly significant. However, the “health” concept is not the same from the common man and yogi’s point of view. Yogis do not set the goal of becoming just healthy, the goal for them is rather samādhi. Therefore, the ritual of internal and external fire for the gr̥hasthas, many tantrikas and for the sannyasins has different levels. The yogi awakens the internal fire only in the suṣumnā, “dying for the outside world.” Initially, nāda sounds appear with the fire and light, but with full realisation, even the smallest fluctuations of prāṇa-spandana are extinguished. The above-mentioned Maitri (Maitrāyaṇīya) Upaniṣad (6.22) says of two kinds of Brahman: Śābda Brahman, who manifests himself in the sound and Para Brahman, where the sound disappears.