Sotah 9:4

Rabbi Eliezer says they measure from the body’s navel; Rabbi Akiva says from the nose. Rabbi Eliezer says they measure from the neck, which is the place from where a person is rendered a corpse.

Sotah 9:5

After the elders of Jerusalem departed, the elders of the city brought a heifer that was never worked or drawn with a yoke; it need not be unblemished. They brought the calf to a valley of hard rock, which is the meaning of the word eisan in Deuteronomy 21:4. If the valley is not rock hard, the ceremony is nevertheless effective. They break the calf’s neck from behind using a cleaver. The site where this ceremony is performed may never be planted or plowed but one could use the site to comb flax or to chisel stone.