Jaiminīya (Talavakāra) Brāhmaṇa 58.318

For night is subsequent to day.
He saw this ninefold Agniṣṭoma.
He deposited it, this very fire, upon this earth.
With that light he saw these worlds.
He extended these eight Ukthyas along the eightfold cattle, the directions; the Vāmadevya [as] the atmosphere;
Āditya indeed as the tenth day, and yonder world as the subsequent Atirātra.
He attained this reach.
He became this all.
He attains this very reach and becomes this all, who knows thus.
The six-day Pṛṣṭhya rite is indeed the seasons; the Chandomas are the directions; these two worlds are the two Atirātras.
The six-day Pṛṣṭhya rite is indeed the seasons; the half-months, months, and days and nights are the Chandomas;
these two worlds are the two Atirātras.
Now, the two Agniṣṭomas which are in between, they are Indra and Agni.
Since, being nearer, the earlier day has a smaller stoma, therefore this Agni, being nearer, burns more gently.
And since, being farther away, the final day has a larger stoma, therefore yonder Āditya, being farther away, burns more fiercely.
He who knows the entire twelve-day rite as corresponding to the self, all desires accrue to him.

The prior Atirātra is the counterpart of the feet.

There are nine breaths in a person.

That is the Trivṛt [stoma].

The introductory day is trivṛt.

Thereby it is corresponded to.

Ten are the fingers of the hands, and four are the limbs of the chest and arms.

What is above the navel, that is fifteenfold.

The second day is the fifteenfold [stoma].

Thereby it is corresponded to.

Ten are the fingers of the hands; seven are the breaths in the head.

That makes seventeen.

The third day is seventeenfold; thereby it is corresponded to.

Ten are the fingers of the hands, ten are the toes of the feet, and the trunk is the twenty-first.

The fourth day is twenty-onefold.

Thereby it is corresponded to.