I frequently they went down a road with a high fence on either side. At this point the Indian had a most peculiar experience. He began to squirm and twist and wriggle in his saddle. The Confederate officer asked him what was the matter. He explained that some kind of bug had bitten him right in the middle of the back, and he was crazy with the desire to scratch. His contortions were so passive that the officer began to scratch himself. Finally, he told the Indian, who had been disarmed, to scratch and he did with it. The Indian had a keen, cleverly concealed in his saddle. He carefully scratched around the fence, drew it, shot the officer's mount. Jumped his own over the fence, and escaped, because there was no other horse in the detachment that could follow.
Mark Spreckels' Study Here. 