Backbone
Srimad-Bhagavatam
SB Canto 2
SB 2.7.13, Purport:
The gigantic work of churning the milk ocean by the combined effort of the demigods and the demons required a gigantic resting ground or pivot for the gigantic Mandara Hill. Thus to help the attempt of the demigods the primeval Lord assumed the incarnation of a gigantic tortoise, swimming in the ocean of milk. At the same time, the mountain scratched His backbone as He was partially sleeping and thus relieved His itching sensation.