The Luz Bone:^[1]

There is a certain bone in the body called the Luz bone which does not benefit or nurture from any food other than the food eaten during Melaveh Malka.^[2] This bone is created from the essence of mans seed and cannot be destroyed, not in fire, nor through a grinder, nor through erosion. It lasts forever and is what Hashem will use to build the body by the resurrection. The reason this bone is everlasting is because it did not benefit from the tree of good and evil.

Where is the Luz Bone?

The Midrash[3] states that the Luz bone is in the spine. Some[4]  write it is the C1 vertebrae at the top of the cervical spine. This refers to the most top bone that is found by the neck, by the start of the spine, right under the end of the bone of the skull. Others write that it is the coccyx bone, which is a small triangular bone at the base of the spinal column in humans, formed of fused end of the spinal cord. It is below the 18 spinal disks.[5] Others say that according to the Kabbalists it is the bone at the back of the skull by which one places the knot of the Tefillin. This bone is found right above the C1 vertebrae, as hold the first opinion.[6] Others write that it is a small tiny round bone not larger than the size of a barley, that is found under the brain of man within the skull, and contains veins in the form of a spider web.[7]

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[1] Beis Yosef 300:1; Shibulei Haleket 130; Mateh Moshe 513; Elya Raba 300:2-3; brought in Kaf Hachaim 300:1; Ketzos Hashulchan 100 footnote 7

[2] See Lekutei Sichos 36 Beshalach for the reason why the Luz bone benefits specifically from the meal of Melaveh Malka

[3] Rebbe Yehoshua in Koheles Raba 12:5; Bereishis Raba 28:3; See Tosafus Bava Kama 16a and Bereishis Raba 56:2

[4] Midrash Tanchuma Chayeh Sara 7

[5] Aruch “Luz”; Rashi on Bereishis Raba 28:3; Avodas Hakodesh 2:40; Avkas Rochel 2:4

[6] Likkutei Nach Arizal Parshas Shoftim; Likkutei Shas on Rosh Hashanah; Sefer Hamamarim 5711 p. 209

[7] Mavor Yabok Sefas Emes 6 p. 98 in name of Ramak