975. Once formed, the conservation of the proper proportions of the composition of the blood is effected by the excretory processes already described; by the removal of its superfluous water by the lungs, skin, and kidneys; by the removal of its superfluous carbon, azote, and oxygen by the lungs, liver, and kidneys; by the removal of saline and mineral matters chiefly by the kidneys; and finally by the instantaneous removal of products of decomposition formed in the course of the organic actions, chiefly, it would appear, by the kidneys. 976. Once formed, and duly concentrated and purified, the blood is sent out by the left heart to the system. Driven by the heart through the main trunks and branches of the aorta, the blood ultimately reaches the capillary arteries, which do not divide and subdivide indefinitely, but ultimately reach a point beyond which they no longer diminish in size. Not all of the same magnitude, some are large enough to admit of three or four of the red particles of the blood abreast; the diameter of others is only sufficient to admit of two or even of one; others are capable of transmitting only the clear and transparent liquor sanguinis; while in many cases the membranous tunics of the capillaries wholly disappear; the blood no longer flows in actual vessels, but is contained in the substance of the tissues in channels which it forms in them for itself (304). 977. Under the microscope, says Müller, the blood corpuscles are seen distinctly pouring from the smallest ramifying arteries into vessels which grow no smaller. After leaving these, they again assemble in the origins of veins formed in collected branches. The blood corpuscles flow in the finest capillaries, one after another, and often interruptedly. They are colourless when they flow singly; accumulated more thickly, they appear yellow, and in still greater quantity, yellowish red or red. In animals that have lost their strength, the globules flow without stoppage: when the animal is weak and the motion is retarded, the globules move by starts; they move on, but go more rapidly by fits. In a still weaker animal they only advance during the impulse of the heart, and then fall back a little. When several arterial currents unite in an anastomosis, one current always predominates and traverses the anastomosis alone, to mingle its blood in the other currents. Thus the currents meet and divide in the reticulate capillaries till all are collected again in veins. Sometimes the direction of the current changes, when another current becomes stronger, and the previous leader weaker, according to the pressure exerted on the part.