וַיֹּ֨אמֶר יְהוָ֜ה לָ֗הּ שְׁנֵ֤י גיים [גוֹיִם֙] בְּבִטְנֵ֔ךְ וּשְׁנֵ֣י לְאֻמִּ֔ים מִמֵּעַ֖יִךְ יִפָּרֵ֑דוּ וּלְאֹם֙ מִלְאֹ֣ם יֶֽאֱמָ֔ץ וְרַ֖ב יַעֲבֹ֥ד צָעִֽיר׃
And the LORD said unto her: Two nations are in thy womb, And two peoples shall be separated from thy bowels; And the one people shall be stronger than the other people; And the elder shall serve the younger.
Zohar
"And Hashem said to her, 'Two nations are in your womb, and two peoples'" (Gen. 25:23). These are the two proud ones, the liver and the heart. Rabbi Yosi said that these are the brain and the heart, but Rabbi Yehuda said, The brain is not included in this, for it is written, "in your womb (lit. 'belly')," and the brain is not in the belly, but in the head. "And two peoples from your bowels and the elder (lit. 'great') shall serve the younger." This is the liver, which is great and big, and which serves the heart, as Rabbi Yehuda said, The liver receives the blood, and serves it to the heart!