This system commences in the skin, and in the little spaces between the elements of the tissues in almost all parts of the body. It can be easily demonstrated by means of injections, for if a needle with a fine bore be plunged into the skin or into the muscles, and mercury or warm solution of gelatine holding some colouring material in suspension, as vermilion or Prussian blue, be forced through it with syringe, a beautiful and very close net-work of channels comes into view, which is quite distinct from the blood - vessels, and has no direct communication with them. The minuter blood - vessels, although they come into very close relation with the cells of glands and the fibres of muscle, do not actually touch them. There is always a layer of fluid, named lymph, between the two, so that, separating the blood from the actual constituents of every tissue, there are the wall of the blood-vessel and the layer of lymph outside that wall, as well as the walls of the vessels in which the lymph is contained, which indeed consists only of very thin cells. It is in the irregular spaces that are thus formed that the lymphatics arise. The spaces thus lined by flat cells soon, instead of being irregular, become tubular.