The surplus leaves the limb by the lymphatic vessels. A gland is never at rest. In the intervals between the ejection of its secretion its cells are preparing materials for the next outflow. Lymph is always flowing from a gland; its amount increases as the activity of the gland increases. More lymph leaves the blood when the gland is exceptionally active than when it is relatively quiet. Some of it is not reabsorbed into the blood. A certain proportion of the waste products of the active gland are hurried away by the overflow system in the direction of the thoracic duct. Lymph is the reservoir of nutriment upon which every cell in the body draws. It is improbable that in health and under normal conditions the activity of any organ is ever restricted for want of sufficient food. As food is removed from lymph, it is instantly replaced by fresh food from the blood. There is some evidence—not very clear—that the removal of waste products offers greater difficulty than the renewal of supplies of food. When the activity of muscles has been excessively prolonged they ache. It has been supposed that their unwillingness to do more work is due, not to the exhaustion of the food which they use up when contracting, but to the inadequacy of the lymph and blood to carry off all refuse. This, at least, is the explanation of fatigue which is usually offered, although it is difficult to understand why the arrangements for removing waste products which have worked to perfection for eight hours should during the ninth hour become rapidly ineffective.