Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.3.16

3.8.3.16

hṛdayamu vai paśuḥ | tadasyātmana evāgre 'vadyati tasmādyadi kiṃcidavadānaṃ hīyeta na tadādriyeta sarvasya haivāsya tatpaśoravattam bhavati yaddhṛdayasyāgre 'vadyati tasmānmadhyataḥ sato hṛdayasyaivāgre 'vadyatyatha yathāpūrvam

The heart, then, is the animal; thus he first makes a portion of its very self (or soul). And, accordingly, if any portion were omitted, he need not heed this, since it is of his entire animal victim that the first portion is made which is made of the heart. He therefore first makes a portion of the heart, that being in the middle. Thereupon according to the proper order.