Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 2.2.3

2.2.3

tametāḥ saptākṣitaya upatiṣṭhante tadyā imā akṣanlohinyo rājayastābhirenam̐ rudro'nvāyatto'tha yā akṣannāpastābhiḥ parjanyo yā kanīnakā tayādityo yatkṛṣṇaṃ tenāgniryacchuklaṃ tenendro'dharayainaṃ vartanyā pṛthivyanvāyattā dyauruttarayā nāsyānnaṃ kṣīyate ya evaṃ veda2 tadeṣa śloko bhavati arvāgbilaścamasa ūrdhvabudhnastasminyaśo nihitaṃ viśvarūpam | tasyāsata ṣayaḥ sapta tīre vāgaṣṭamī brahmaṇā saṃvidāneti | arvāgbilaścamasa ūrdhvabudhna itīdaṃ tacchira eṣa hyarvāgbilaścamasa ūrdhvabudhnastasminyaśo nihitaṃ viśvarūpamiti prāṇā vai yaśo viśvarūpaṃ prāṇānetadāha tasyāsata ṣayaḥ sapta tīra iti prāṇā vā ṣayaḥ prāṇānetadāha vāgaṣṭamī brahmaṇā saṃvidāneti vāgdhyaṣṭamī brahmaṇā saṃvitte

Then the seven imperishable ones approach him. There are the red lines in the eye, and by them Rudra clings to him. There is the water in the eye, and by it Parganya clings to him. There is the pupil, and by it Âditya (sun) clings to him, There is the dark iris, and by it Agni clings to him. There is the white eye-ball, and by it Indra, clings to him. With the lower eye-lash the earth, with the upper eye-lash the heaven clings to him. He who knows this, his food does never perish.

On this there is this Sloka:
'There is a cup having its mouth below and its bottom above. Manifold glory has been placed into it. On its lip sit the seven Rishis, the tongue as the eighth communicates with Brahman.' What is called the cup having its mouth below and its bottom above is this head, for its mouth (the mouth) is below, its bottom (the skull) is above. When it is said that manifold glory has been placed into it, the senses verily are manifold glory, and he therefore means the senses. When he says that the seven Rishis sit on its lip, the Rishis are verily the (active) senses, and he means the senses. And when he says that the tongue as the eighth communicates with Brahman, it is because the tongue, as the eighth, does communicate with Brahman.