Besides the skeletal muscles there is active functional metabolism in the various secreting glands; a feature of their activity is that it must be very great at certain times, but falls to little or nothing at others; thus during the actual taking and digesting of food the various glands which secrete digestive juices are exceedingly active, but in the intervals they may be doing little or no work. They require a very copious blood supply when they are functioning, but need very little between times. The blood vessels flowing through all these glands are under the influence of the vasodilator center, so that they are opened as widely as possible while the glands are active. They, therefore, receive much more blood in proportion to their size than they would if it were not for the action of this center. Between times their blood supply falls off to that which suffices for inactive tissues generally. In time of emergency the action of the vasoconstrictor center upon the vessels through these glands is such as to cut off the blood supply to them almost completely. This is well illustrated in the dry mouth of the frightened man, showing that the salivary glands have suspended activity, a suspension due largely, if not wholly, to the cutting down of the blood supply through them. In the section just completed we have tried to give some idea of the way in which the circulating blood provides the various tissues with the materials they require and is adjusted automatically to meet variations in demand from different tissues. Just one more point needs to be noted in completing the account of the conveyer system. The tissue fluids which serve as the connecting links between the circulating blood and the individual cells are necessarily full of waste products, because these come directly from the cells to the tissue fluids and afterward pass on from them to the blood. The result is that the cells are constantly bathed in a solution of their own waste products. There is only one way in which relief from this condition can be obtained and that is by moving the used tissue fluid away bodily and letting it be replaced by fresh fluid. As a matter of fact, this happens; there is an oozing of fluid through the walls of the capillaries from the blood into the tissue spaces; this of course pushes the fluid already in those spaces on ahead of it. If there were no place to which the fluid could go, the result would be a swelling, as the tissue became more and more filled with fluid. This is avoided ordinarily by a drainage system whereby the tissue fluids are carried off as fast as more fluid comes in from the blood, but when liquid is poured out faster than it will drain off, as from a bruise or wrench, we do get a swelling.