The head
Aries
The 1st sign, ruled by Mars
In the cosmic person, Mars is strength, and rules the marrow.
"The twelve signs of the zodiac, beginning at the first point of Aries … are respectively the head, face, breast, heart, belly, navel, abdomen, genital organ, the two thighs, the two knees, the two ankles, and the two feet of Kalapurusha, the cosmic person."
Brihat Jataka 1.4