ब्रह्मणः सोम्य ते पादं ब्रवाणीति ब्रवीतु मे भगवानिति तस्मै होवाच प्राणः कला चक्षुः कला श्रोत्रं कला मनः कलैष वै सोम्य चतुष्कलः पादो ब्रह्मण आयतनवान्नाम ॥ ४.८.३ ॥
brahmaṇaḥ somya te pādaṃ bravāṇīti bravītu me bhagavāniti tasmai hovāca prāṇaḥ kalā cakṣuḥ kalā śrotraṃ kalā manaḥ kalaiṣa vai somya catuṣkalaḥ pādo brahmaṇa āyatanavānnāma || 4.8.3 ||
3. [The madgu said,] ‘O Somya, let me tell you about one foot of Brahman.’ [Satyakāma replied,] ‘Yes, lord, please tell me.’ [The madgu] said to him: ‘Prāṇa is one part, the eyes are another part, the ears are a third part, and the mind is a fourth part. O Somya, these are the four parts that make up a foot of Brahman. This foot is named Āyatanavān, the Support’.
Somya, O Somya; brahmaṇaḥ te pādam bravāṇi iti, I will now tell you about a foot [or quarter] of Brahman; bravītu me bhagavān iti, yes, lord, please tell me; tasmai ha uvāca, [the madgu] said to him; prāṇaḥ kalā, prāṇah [the vital force] is one part; cakṣuḥ kalā, the eyes are one part; śrotram kalā, the ears are one part; manaḥ kalā, the mind is one part; eṣaḥ vai somya catuṣkalaḥ pādaḥ brahmaṇaḥ, O Somya, these are the four parts that make up a foot of Brahman; āyatanavān nāma, named Āyatanavān, the Support.
This part of the story, about the madgu, is devoted to the teaching of prāṇa. In fact, the madgu represents prāṇa.
Here the role of the mind has to be understood. The mind is the fourth part of this foot of Brahman, and it is the repository of all that we experience through our diverse organs. This is why it is called Āyatanavān, the Support.