Besides the little glands in the walls of the intestine there are many thousands of little finger-like projections standing up from the walls. These are called _villi_. Each villus has in it very small vessels, into which the food passes after it has been digested. These vessels are of two sorts: blood vessels, which take up the digested starch and proteid foods, and another sort known as _lymphatics_, which take up the fats. All the lymphatics combine into a single vessel which empties into the great vein at the base of the neck. Thus the fatty foods pass into the blood and are mingled with the food materials taken up directly by the blood vessels. CIRCULATION OF THE BLOOD [Sidenote: Two kinds of blood vessels] There are two kinds of blood vessels in the body. We call them _arteries_ and _veins_. The arteries serve to carry the blood from the heart to all parts of the body, and the veins serve to carry the blood back to the heart. The heart is really a part of the blood vessels, half of each side being like the veins and half like the arteries. [Sidenote: Arteries and veins] The walls of the arteries are thicker than those of the veins. Two sets of arteries leave the heart, one from each side. The artery that starts from the right side of the heart goes to the lungs and carries _venous blood_, which has a very poor supply of oxygen and is full of impurities. We call this the _pulmonary artery_. The artery that leaves the left side of the heart goes to all parts of the body but the lungs and carries _arterial blood_, which has much more oxygen and is much more free from impurities. [Sidenote: Capillaries] If we follow the blood as it circulates we will see how it reaches all parts of the body. The big artery that leaves the left side of the heart divides into smaller and smaller arteries until there are branches going to every part of the body. These branches keep dividing until they are so small that we call them _arterioles_, and these little arterioles again divide and become so small that we call them _capillaries_. [Sidenote: Changes in the blood] While the blood is passing through the arterioles and the capillaries, something is happening to it. The little cells with which these small vessels come in contact have been taking the oxygen and the nourishing material out of the blood. At the same time they have been putting something into the blood. In place of the oxygen they have been putting in carbon dioxid and in place of the nourishing material they have been putting in the worn-out materials from the cells. As the blood passes through certain parts of the body, such as the kidneys, the worn-out materials from the cells are taken out of the blood and sent out of the body. The carbon dioxid is left in the blood until it goes to the lungs. [Illustration: FIG. 83. The white vessels represent the circulation of the arterial blood; the gray, the circulation of the venous blood.] [Sidenote: Venous blood]