Muṇḍaka Upaniṣad · 2.2.6

अरा इव रथनाभौ संहता यत्र नाड्यः । स एषोऽन्तश्चरते बहुधा जायमानः । ओमित्येवं ध्यायथ आत्मानं स्वस्ति वः पाराय तमसः परस्तात्

arā iva ratha-nābhau saṃhatā yatra nāḍyaḥ | sa eṣo'ntaś carate bahudhā jāyamānaḥ | om ity evaṃ dhyāyatha ātmānaṃ svasti vaḥ pārāya tamasas parastāt

As the spokes in the hub of a wheel, thus gathered are the nāḍīs where that (Self) moves internally in multiple ways, being born. Meditate thus upon the Self with Om. Hail to you, toward the other shore beyond darkness.

The metaphor of the subtle body: the nāḍīs as channels of vital energy.

Arāḥ — the spokes. Multiple, divergent, but united in a center.

Ratha-nābhi — the hub of a wheel. The center where all spokes converge.

Nāḍyaḥ — subtle-energy channels. The system of vital circulation in the body.

Sa eṣaḥ antaḥ carate — that moves internally. The prāṇa, vital energy.

Bahudhā jāyamānaḥ — being born in multiple ways. The various forms of vital expression.

Om iti evaṃ dhyāyatha — meditate thus with Om. The mantra as vehicle of contemplation.

Svasti vaḥ — hail to you. Blessing of well-being.

Pārāya tamasas parastāt — toward the other shore, beyond darkness. Liberation as crossing.

Prāṇāyāma and meditation work with these nāḍīs toward the center.