This ubiquitous "connective tissue" underlying the skin, pushing its way into and around every part of every structure in the body, is the "source"--the reservoir, as it were--from which the lymph stream and the finest lymphatic vessels take their origin. The question may very naturally be asked, "How is it that the lymph flows along the channels provided by the transparent lymph vessels and is poured through 'the thoracic duct' into the great vein near the heart?" If we inject a suitable coloured fluid by means of a needle-pointed syringe into any mass of connective tissue, we can see the fluid pass into the numerous lymph vessels previously invisible, and if we inject into them a weak solution of silver nitrate we can, subsequently by aid of the microscope, make out the structure of the walls of the lymphatics and the lining pavement cells which become stained of a brown colour by the silver when exposed to light. But there is no muscular envelope, nothing like "a lymph-heart" in mammals, to drive the lymph along. There are valves or flexible flaps in the walls of the lymph-vessels, as there are in the veins, and the lymph is driven to the heart by the intermittent pressure upon these valved tubes, caused by the movements of the muscles and of the body generally. The valves, like those of the veins, prevent the flow of the lymph backwards, but allow it to pass forward towards the heart. This is shown by the examination of a narcotized mammal (killed immediately after the examination has been made). A glass tube is placed in the thoracic duct, and about a dozen drops of lymph (which would have been delivered into the great vein) pass from it in a minute. If, however, the animal's legs are moved, as though in running, or if "massage" is applied to the limbs--the pressure being directed from the extremities towards the heart--then a greatly increased flow of lymph is observed, as much as sixty drops in a minute! This is the chief explanation of the value to our health of exercise, and also of the importance of "massage" as a treatment in disease. Either exercise or massage entirely revolutionizes the rate of flow of the lymph, quickening it so greatly that the physiological effect on the general chemical processes going on in the body cannot fail to be most important.