VASCULAR SYSTEM. The unicellular organism receives all the supplies necessary for its growth and maintenance directly from its environment, either by diffusion of substances held in solution, or by direct intake of foodstuffs, which undergo chem ical disintegration in vacuoles containing liquid into which the necessary enzymes are secreted. In the multicellular organism, such direct intake of foodstuffs by diffusion by every individual cell is impossible, for only the outer layer of cells is in contact with the environment, and this layer is usually protected by some form of membrane which is impermeable to water and food substances.
The vascular system has been developed to convey nutrient materials, etc., to the various parts of the organism, and the blood serves as intermediary between the environment and the organs of the body. This function can be fulfilled only if the blood is in continuous circulation, carrying nutrient materials and oxygen to the tissues, and conveying the waste products of metabolism from the tissues to the places where they are excreted. The heart is the organ which provides the necessary energy for this circulation, and the blood vessels are the channels which convey the blood to and from the tissues.
In the case of most invertebrate animals, the blood or analogous fluid is not all enclosed in blood vessels, and the various tis sues lie freely in the fluid ; which is kept in constant motion. In
the higher animals, however, the blood is entirely enclosed in a system of tubes, and the nutrient substances, etc., are brought into contact with the cells only after their diffusion through the thin walls of the finest blood vessels. By the force of the cardiac contraction, the blood is driven through the tissues by way of thick-walled tubes, the arteries, and back to the heart by a system of thinner-walled vessels, the veins. In the tissues, the blood passes through a fine meshwork of capillaries, the walls of which consist of a single layer of delicate cells, which allow a free inter change of material to take place between the blood within and the tissue fluids outside the vessel.
In fishes (fig. 1, A) the heart consists of one auricle and one ventricle. The blood is received from the great veins into the auricle. By the contraction of the auricle the blood is forced into the ventricle and this, when it contracts, sends the blood on to the bulbus arteriosus. The blood passes through the bran chial arteries into the gills, where it takes up oxygen, and then flows on into the aorta, by which