Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.4.1

3.4.4.1

grīvā vai yajñasyopasadaḥ śiraḥ pravargyaḥ | tasmādyadi pravargyavānbhavati pravargyeṇa pracaryāthopasadbhiḥ pracaranti tadgrīvāḥ pratidadhāti

Verily the Upasads (homages or sieges) are the neck of the sacrifice, and the Pravargya is its head. Hence when it is performed with the Pravargya, they perform the Upasads after performing the Pravargya, and thereby they put the neck in its place.