The Yellow Emperor asked, "I would like to hear the difference between the depth and shallowness of the thorns." Qi Bo against the Sun: The bone-piercing does not harm the tendons, the pricking of the tendons does not harm the flesh, the pricking of the flesh does not harm the veins, the pricking of the veins does not harm the skin, the peeling of the skin does not harm the flesh, the pricking of the flesh does not harm the tendons, and the sharpening of the tendons does not harm the bones. The emperor said, "I have no idea what it means. I am willing to hear its explanation." Read Bo said: "When the needle pierces the bone but does not hurt the sinew, it will reach the sinew but not the bone." When a tendon is pricked but does not harm the flesh, it reaches the flesh but does not reach the tendon. When the flesh is sharp but does not harm the meridians, it reaches the meridians and does not reach the flesh. When regulating the pulse without damaging the skin, it reaches the skin and does not reach the pulse. The saying goes that when a needle is inserted into the skin without harming the flesh, the disease lies in the skin. The needle is inserted into the skin without harming the flesh. If the meat is pierced without damaging the tendons, it is passed through the tendons. If stabbing the tendons does not harm the bones, it means passing through the bones within the tendons. This is what is called "reversal".