The meridian-collateral system of traditional Chinese medicine refers to the regulating system composed of the known loose connective tissue, liquid-Qi of tissue, energy substances, nerves, blood vessels, lymph, etc. existing in the fascia space between the skin, muscles and bone and some associated unknown synthetic functions. This hypothesis of meridian-collateral, Qi-passage, and fascia space composed of multiple substances is probably able to unify different viewpoints about that the meridian-collateral system is in fact an integrated substance of nerves, body fluid and energy. In the present paper, the authors introduce their viewpoints about the essence of meridian-collaterals from nerves, humor and energy, and unknown function of the known anatomical structure.