Śatapatha Brāhmaṇa ("of a hundred paths") 8.4.4.4

8.4.4.4

sa eṣa paśuryadagniḥ | so 'traiva sarvaḥ kṛtsnaḥ saṃskṛtastasya trivṛdvatyāveva śiraste yattrivṛdvatyau bhavatastrivṛddhi śiro dve bhavato dvikapālaṃ hi śiraḥ pūrvārdha upadadhāti purastāddhīdaṃ śiraḥ

Now, this Agni (fire-altar) is an animal, and he is made up (restored) here whole and entire. His head is the two (bricks) containing the Trivrit; and as to why these two are such as contain the Trivrit,--the head is threefold (trivrit). There are two of them, because the head consists of two bones (kapâla). He lays them down in front, for this head is in the front (of the animal).