Śatapatha Brāhmaṇa ("of a hundred paths") 8.3.4.4

8.3.4.4

yadveva sapta purastādupadadhāti | sapta vā ime purastātprāṇāścatvāri dorbāhavāṇi śiro grīvā yadūrdhvaṃ nābhestatsaptamamaṅge 'ṅge hi prāṇa ete vai sapta purastātprāṇāstānasminnetaddadhāti

And, again, as to why he places seven in front,--there are seven (channels of the) vital airs here in the front part (of the animal) --the four upper and lower parts of the fore-feet, the head, the neck, and what is above the navel that is the sixth, for in each limb there is a vital air: this makes seven vital airs here in front; it is them he thus lays into him (Agni-Pragâpati).