intestines is obliged to pass again through the capillaries of the liver before going on into the main stream of the circulation. This is an important feature of the renewal of the food supplies of the blood. We have now traced the blood from the heart through the body back to the heart again, and have seen how in its course some of it will pass through such active tissues as muscles or brain or glands, so that the interchanges can go on by which the fluids in these active tissues can take up needed materials and give off wastes. Also a part of it flows through the digestive organs, where food materials can be taken up, and another part flows into the kidneys, where wastes can be gotten rid of. This leaves us to consider only the passage of the blood through the lungs, where the supply of oxygen is to be taken up and the gaseous waste product, carbon dioxide, is to be disposed of. The most urgent requirement of the body is the requirement for oxygen. There is under ordinary circumstances at all times some surplus of food materials stored in the cells, so that even though the renewal of their surrounding fluids from the blood should stop, they could keep on going for a time on the material that is stored within them; but there is no such storage of oxygen. The cells in the body lead almost a hand-to-mouth existence so far as their oxygen supply is concerned. They are constantly withdrawing oxygen from the tissue fluids surrounding them, and these fluids are just as constantly withdrawing it from the blood; therefore any failure of the blood to be properly supplied with oxygen results very promptly in a condition of oxygen hunger in the cells. This means prompt cutting off of metabolism, since metabolism is a matter of oxidizing fuel, and oxidation cannot go on unless the oxygen is provided. This urgent need for oxygen is met in the body by having the arrangement for supplying it to the blood much more perfect than the other renewal arrangements. We saw a moment ago that only part of the whole blood stream passes through the digestive organs at any given time and only part of the stream passes through the kidneys. The whole stream, on the other hand, passes through the capillaries of the lungs. This is brought about by having an arrangement whereby the combined venæ cavæ after entering the heart communicate with an outlet in the form of an artery leading to the lungs. This artery, which is called the _pulmonary artery_, breaks up into capillaries in the lungs, which reunite into the _pulmonary vein_ which comes back to the heart again. It is from the pulmonary vein that connection is made with the aorta, starting the blood on its course through the body again. We see then that the blood passes through the heart twice in each complete round, once as it comes in from the body at large on its way to the lungs, and again as it comes in from the lungs on its way to the body at large.