The works of Ādi Śaṅkarācārya

Chāndogyopaniṣadbhāṣyam

Verse 295

तेजोऽशितं त्रेधा विधीयते तस्य यः स्थविष्ठो धातुस्तदस्थि भवति यो मध्यमः स मज्जा योऽणिष्ठः सा वाक् ॥ ३ ॥

In plain words

Consumed fire is divided in three ways. Its grossest part becomes bone, its middle part becomes marrow, and its subtlest part becomes speech.

Close to the Sanskrit

Fire eaten is divided in three ways. Of it, whatever is the grossest constituent, that bone becomes; whatever is the middle, that is marrow; whatever is the subtlest, that is speech. 3