Verse 3.12.7-9

यद्वै तद्ब्रह्मेतीदं वाव तद्योयं बहिर्धा पुरुषादाकाशो यो वै स बहिर्धा पुरुषादाकाशः ॥ ३.१२.७ ॥
अयं वाव स योऽयमन्तः पुरुष अकाशो यो वै सोऽन्तः पुरुष आकाशः ॥ ३.१२.८ ॥
अयं वाव स योऽयमन्तर्हृदय आकाशस्तदेतत्पूर्णमप्रवर्ति पूर्णमप्रवर्तिनींश्रियं लभते य एवं वेद ॥ ३.१२.९ ॥
॥ इति द्वादशः खण्डः ॥

yadvai tadbrahmetīdaṃ vāva tadyoyaṃ bahirdhā puruṣādākāśo yo vai sa bahirdhā puruṣādākāśaḥ || 3.12.7 ||
ayaṃ vāva sa yo'yamantaḥ puruṣa akāśo yo vai so'ntaḥ puruṣa ākāśaḥ || 3.12.8 ||
ayaṃ vāva sa yo'yamantarhṛdaya ākāśastadetatpūrṇamapravarti pūrṇamapravartinīṃśriyaṃ labhate ya evaṃ veda || 3.12.9 ||
|| iti dvādaśaḥ khaṇḍaḥ ||

7-9. That which is Brahman is also the space outside the body. That which is the space outside the body is also the space inside the body. And that which is the space inside the body is also the space within the heart. That treasure within the heart is full and unchanging. He who knows this is always full and not subject to change.

Word-for-word explanation:

Yat vai tat brahma iti, that which is Brahman; idam vāva tat, it is that; yaḥ, which; ayam, is this; ākāśaḥ, space; bahirdhā puruṣāt, outside the human body; yaḥ vai saḥ ākāśaḥ, that which is the space; bahirdhā puruṣāt, outside the human body; ayam vāva saḥ, it is that; yaḥ ayam ākāśaḥ, which is this space; antaḥ puruṣe, inside the human body; yaḥ vai saḥ antaḥ puruṣe ākāśaḥ, that which is the space inside the human body; ayam vāva saḥ, it is that; yaḥ ayam ākāśaḥ, which is this space; antaḥ hṛdaye, inside the heart; tat etat, it is that; pūrṇam, full; apravarti, unchanging; śriyam, treasure; yaḥ evam veda, he who knows this; pūrṇām apravartinīm labhate, becomes full and is not subject to change. Iti dvādaśaḥ khaṇḍaḥ, here ends the twelfth section.

Commentary:

Earlier Brahman was described as the gāyatrī with four quarters and also as amṛta, nectar. Here it is described as space. It is the same space which is inside as well as outside the human body, and also within the heart. The Upaniṣad says that this ākāśa in the heart is full and never changes. That is to say, it is free from desire and because of that it is never subject to change. Anyone who knows that ākāśa in the heart as such is himself always full and free from desire.