The blood flows through the body in a system of tubes, or _blood-vessels_, which branch repeatedly, and at last become merged in the microscopic _capillary blood-vessels_. These capillaries are present in nearly all parts of the body except the epidermis and epidermal structures (hairs, feathers, scales, etc.). They have exceedingly thin walls, which present no resistance to the passage of the nutritious substances contained in the blood, so that these can be absorbed by those parts of the body which lie between the individual capillary vessels. The central organ of the circulation is the _heart_, an enlarged part of the vascular system, possessing thick muscular walls. By contraction of these, the blood is driven out of the heart (Fig. 6, _H_); and its exit is possible on one side only (_a_), as at the other side (_b_) there is a valve, which closes when the heart contracts. The vessel into which the blood leaving the heart enters is termed an _artery_ (_S.A._) It divides into several branches, also known as arteries, and the smallest arteries pass into capillaries, which again are connected with _veins_, which join larger and larger veins, until finally one or a few open into the heart (_A_). [Illustration: FIG. 6.—Diagram of the Course of the Circulation. ] Since the blood in the course of its circulation gives up some of its nutriment to the various parts of the body, it would in the end become useless for the purposes of nutrition if it did not receive a fresh supply of food-stuffs from the gut, either directly or indirectly (through the lacteal system). But apart from this, the blood would ultimately become useless, and that very quickly, if it did not traverse the lungs, kidneys, and sweat-glands. It is well known to every one that a man or animal cannot live without air, or at any rate without a certain gas, _oxygen_, that is contained in air. This oxygen must be able to penetrate into the minutest particles of the body, and the blood, in the corpuscles of which it is contained, carries it everywhere. In the smallest particles (molecules) of the body an oxidation (combustion) of body substance takes place, which not only causes an evolution of heat, but also renders the body capable of doing work. But if now the blood passes from the capillaries into the veins, it contains too little oxygen. And besides, it has taken up from the molecules of the body several substances, developed in those molecules, which would be fatal to the animal if they were not removed from the body. Now, when the blood streams through the lungs, it gets rid of the poisonous gaseous matter, and when it traverses the kidneys and sweat-glands it parts with the injurious liquid and solid substances. But in the lungs the blood takes up at the same time fresh oxygen; and since in this way the air in the lungs becomes poor in oxygen, the _movements of breathing_ (respiration) provide for the passage of a fresh supply of oxygen into the lungs.