=Blood.=—A portion of the body fluid is enclosed within vessels and kept in circulation by the heart. The heart pumps blood into the aorta. This trunk gives off large arteries, which in turn divide until the finest capillary vessels are reached. The capillary tubes reunite to form veins, which, with the exception of those which collect food from the digestive organs, convey the blood right back to the heart. The veins which drain the stomach and intestines (the organs in which food is prepared for absorption) and the spleen (the organ in which worn-out red blood-corpuscles are in a sort digested) break up in the liver into a second set of small vessels. The pseudo-capillary vessels of the liver reunite to form the hepatic veins, which add the blood that has passed through that organ to the rest of the blood which is passing up the inferior vena cava to the heart. A second capillary circulation is found in the kidney also. The heart is four-chambered (Fig. 10). Its left ventricle drives the blood round the systemic or greater circulation, the blood returning to the right auricle. The right ventricle drives the blood through the lesser or pulmonary circulation, from which it returns to the left auricle. The walls of all bloodvessels, except capillary tubes, are sufficiently thick to prevent the escape of any of the constituents of blood. To support the pressure of the blood which they contain, the arteries and the larger veins need walls of considerable thickness. The walls of the capillaries allow an interchange between blood and lymph in the manner already described (_cf._ p. 39).