Based on the sancai (literally three talents referring to "earth", "human" and "heaven") and wuti (literally five organs referring to skin, muscle, vessel, tendon and bone, which correspond to five internal organs: lungs, spleen, heart, liver and kidney respectively) in Neijing (Internal Classic), as well as the tissue structure theory of modern anatomy, and in combination with the clinical practice of acupuncture by the author in the past over 30 years, it is proposed that four layers are divided in needling depth of acupuncture. The different needling sensations are induced at different layers. The first layer is the skin. When the needle is inserted into the skin, the main sensation is "pain". The second layer is the subcutaneous layer (superficial fascia). The needle tip at this layer generates a strong "swelling" sensation, which radiates along a certain direction. The third layer is the deep fascia and muscle layer. If the needle tip touches the deep fascia, there is a "heavy" feeling. If the needle tip goes into the muscle, an "acid" sensation will occur. The fourth layer is the periosteum and bone layer, which produces a strong sensation of "pain" when touched by the needle tip. In addition, each layer has blood vessels and nerve distribution. When the needle tip touches the blood vessels, a sense of "pain" happens. If the needle tip touches the nerve, there will be a sensation of numbness, which spreads along the direction of nerve conduction. On the contrary, through the patient's needling sensations, we can roughly know which layer and direction the needle tip goes. Better understanding of the relationship between the needling layers and needling sensations contributes to the further study of needling techniques, therapeutic effects of acupuncture and patient's sensations. It is significant for the regulations of needling sensations in patients.
Keywords: Sancai; Wuti; needling layer; needling sensation.