MOST people do not know even of the existence in their own bodies of a fluid called "the lymph," and of a system of vessels and spaces containing it which ramify like the blood-vessels into every part of the body. This arises from the fact that the lymph is translucent and colourless. You can see the finest blood-vessels when the body of a dead rat, sheep, or man is opened, because they are filled with the beautiful red blood, and appear as a rich, coloured network. But the lymph and the lymph-vessels escape notice, and, indeed, are invisible except the largest, because they are colourless. They remained unknown to anatomists long after arteries and veins, and the fine networks of hair-like vessels or capillaries connecting them, were thoroughly well studied. It is, when one thinks of it, a very noteworthy fact, tending to convince us of the readiness with which we may (in the absence of careful examination and attention) overlook the most weighty things, that here is a great system of vessels and spaces in the human body and in that of other animals, carrying on most important operations in our daily life, and yet most of us have never seen any evidence of its existence, and never hold it in our mind's eye as part of the great mechanism of the animal body. The lymph is a clear, colourless fluid, with "corpuscles"--minute nucleated cells or particles of protoplasm--floating in it. The liquid part is closely similar in its properties and chemical constitution to the liquid part of the blood. It, indeed, consists largely of the liquid part of the blood which exudes from the finest hair-like blood-vessels or capillaries as they traverse the various tissues, and it is the chief business of the "lymphatics" or lymph-holding vessels to return this exuded liquid to the blood system, which they do by joining--like the rivulets of a river system--to form two large trunks which open into the great blood-holding veins at the region where they approach the heart. The total amount of lymph in the lymphatic system is difficult to estimate, but it is larger in quantity than the blood in the entire blood-vascular system. A large number of the delicate vessels of the lymphatic system take their origin just below the lining layer of the intestine, and ramify through the transparent membrane, which holds the coils of intestine together, and is called the mesentery. The fatty or oily materials of food pass through the lining "cells" of the intestinal wall into these "lacteal" or milky lymphatics, and consequently in an animal killed and examined after a meal, the fluid in them has a milky appearance, and renders this kind of "lymphatics" visible.