Q. Why is the luz-bone on the spine only nourished from the Melava Malka meal?
A. Midrash (Bereshis Rabbah 18, Koheles 12, see Ta’amei Haminhagim 425) write that the luz-bone is known to be eternal and does not decay, as this is the bone from which the body will be rebuilt at the time of resurrection. The Roman Emperor Hadrian asked Rabi Yoshua Ben Chananiah; how is man to be revived in the world to come, and he replied that it would be from luz, in the back-bone. Hadrian demanded proof and Rabi Yoshua Ben Chananiah took a luz-bone from a spine, and immersed it in water, but it was not softened; he put it into the fire, but it was not consumed; he put it into a mill, but it could not be grounded; he placed it upon an anvil and struck it with a hammer, but only the anvil split and the hammer broke.
The idea of the eternity of the luz is also reflected on the city of Luz, described in Talmud (Soṭah 46b) as the city where the blue dye techeiles is used: the city which Sanncherib entered but could not harm; Nebuchadnezzar came, but could not destroy; the city over which the angel of death has no power; and when the aged are tired of life, they exit its walls and meet their death.
Lebush (300), Rav Yaakov Emdin in his sidur, Mishna Berura (300: 2) mention that it gains its sustenance only from the melave malka meal on Motzei Shabbos. In the future, the resurrection of the dead will begin with this bone.
Shaarei Tzion (ibid.) quotes Eliahu Rabba that explains that the uniqueness of this bone is because it derived no benefit when Adam ate from the Eitz Hadaas; since that was before Motzai Shabbos, so it remained unblemished by that sin. As a result, it is not subject to the same mortality as the rest of the human body
R’ Tzaddok Hakohen and others explain that no matter how low a person has fallen, there exists an indestructible part in him, that can form the basis for a new resurrection; a new life. That is why a person will be resurrected from his luz bone.
He adds that when Leah gave birth to her third son, she named him “Levi”, which shares a root with the word “melaveh.” She said (29: 30), “This time my husband yilaveh – will become attached to me.” Likewise, says R’ Tzaddok, by accompanying the Shabbos on her way, we attach the Shabbos to the week ahead.
The luz also symbolizes the point where physical and spiritual meet. Thus, it is nourished only from melave malka, the meal eaten between the spiritual Shabbos and the material weekday. In a sense it is like the Kosel Hanaaravi: Just as the Western Wall will never be destroyed, and from it the Third Temple will be built, so too the luz can never be destroyed, and from it the person will be re-built during the future resurrection.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a