Sotah, Chapter Nine, Mishnah 4. This Mishnah addresses the question of which point on the body is used as the starting point for measuring, in order to determine which city is closest to the corpse.
From which point on the body do we measure:
"Min hatabur" - from the navel. This is the opinion of Rabbi Eliezer. The Rambam explains that the navel is considered the middle of the body, and it is in this area that the digestive system, where food is processed, is located; therefore we measure from the midpoint.
"Min hachotem" - from the nose. This is the opinion of Rabbi Akiva, since that is the place from which a person draws his breath and his life.
"Min hatzavar" - from the neck. This is the opinion of Rabbi Eliezer ben Yaakov: from the place at which the person became a corpse. The basis for this is the wording of the verse in Yechezkel, "chalelei tzavar" (those slain by the neck), together with the wording of the verse regarding the eglah arufah, "to the city closest to the corpse (chalal)." From this connection between the corpse and the neck, the neck is considered the place where life was primarily centered, and that is where he was killed.
The Gemara in Sanhedrin relates that when Kayin killed Hevel, he did not know how to put him to death. He struck him in various places on his body, until he reached his neck, and there he succeeded in killing him. From here we learn that the neck, according to Rabbi Eliezer ben Yaakov, is the place from which we measure.
In summary: in this Mishnah the Tannaim disagree about the point on the body from which the measurement is taken: Rabbi Eliezer - from the navel, which is the middle of the body and the place where food is processed; Rabbi Akiva - from the nose, the place of the breath of life; and Rabbi Eliezer ben Yaakov - from the neck, the place at which the person became a corpse, as derived from the verses and from the account in the Gemara about Kayin and Hevel.