Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.2.9

6.2.2.9

yadveva saptadaśa | saptadaśo vai puruṣo daśa prāṇāścatvāryaṅgānyātmā pañcadaśo grīvāḥ ṣoḍaśyaḥ śiraḥ saptadaśam puruṣaḥ prajāpatiḥ prajāpatiragniryāvānagniryāvatyasya mātrā tāvataivainametatsaminddhe

And, again, why there are seventeen,--man is seventeenfold,--there are ten vital airs, four limbs, the body the fifteenth, the neck-joints the sixteenth, and the head the seventeenth,--Pragâpati is the Person (or man, purusha), and Pragâpati is Agni: as great as Agni is, as great as is his measure, by so much he thus kindles him.