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HEARTWOOD into the aorta and through its branch. In frogs and toads the heart has three chambers— two auricles and a single ventricle. In reptiles there are three chambers, but the ventricle is divided by an imperfect partition, foreshadowing the four-chambered condition found in higher animals; in the highest reptiles (crocodile) this partition becomes complete. In birds and mammals the heart has two auricles and two ventricles. The right auricle and ventricle receive venous blood and the left ones arterial blood. These facts, based on comparison of the hearts of different animals, become more interesting when it is known that the heart of the highest animal starts, in the embryo, as a single chamber and from that condition passes through all changes up to the four-chambered heart. For example, the heart of the young chick arises after the egg has been incubated two days, and on the third day its beating can be easily seen on opening the egg. At its beginning it is simply a tube with muscular walls formed by the union of two veins. It becomes twisted into an S-shaped, then into a U-shaped tube; it enlarges and the walls get thicker, it then has auricles developed upon it; and by a series of complicated changes it becomes two-, three- and finally four-chambered.

Very early in their development the arteries leaving the heart are built on the fish-like type and undergo a remarkable reduct i on and transformation. These changes furnish clues as to former conditions which help naturalists to read the past history of animals. The heart of the human body also undergoes similar changes. When fully formed, it consists of two auricles and two ventricles, and the right and left cavities are separated by a muscular partition. (See illustration.) The blood is brought from the body to the right auricle and passes through an aperture guarded by valves into the right ventricle; from here it is sent out on the contraction of the walls, through the pulmonary artery, to the lungs. In the lungs it is rendered arterial and passes through the pulmonary veins to the left auricle, thence into the left ventricle and from there into the great aorta, by branches of which it is distributed to all parts of the body—reaching the muscles, glands, brain and all tissues of the body. It then passes through capillaries and enters the veins, by means of which it is brought again to the right side of the heart. The name vein is given to all vessels bringing blood toward the heart, regardless of the quality of the blood, and the artery is used for all blood vessels that carry blood from the heart. It will be at once understood that the pulmonary artery carries venous blood to the lungs and the pulmonary veins arterial blood from the lungs to the heart.

The muscular walls of the left ventricle are very thick, on account of the work devolving upon them of forcing the blood throughout the body. Those of the right ventricle are also thick and strong, but not nearly so thick as those of the left side; their work is merely to send the blood through the lungs. This is called the lesser or pulmonary circulation; the former is the greater or systemic circulation. The valves of the heart between the auricles and ventricles are membranes. There are three flaps on the right and two on the left side. On both sides they are attached to tendonous cords, and these in turn to muscles; by the contraction of the muscles the cords are held tense to prevent the valves from giving way under the pressure when the ventricles contract and from allowing the blood to pass back into the auricles. The beginnings of the pulmonary artery and the aorta are also guarded by bucket-shaped valves that close and prevent the return of blood to the heart as the latter is expanding. The walls of the arteries are very elastic and are put on a stretch when blood is forced into them from the left ventricle. Therefore they exert a steady pressure upon the blood by virtue of their elasticity, and the pulse which is felt in the arteries is lost in the capillaries and not felt in the veins. The veins have valves to aid in holding the blood from receding and to keep it moving toward the heart.

 Heart′wood. As the trunks of trees increase in diameter, new layers of wood are laid down each year upon the outside of the older wood. As the ascending sap moves along through the newer wood, it gradually abandons the older, more interior wood. This wood gradually changes in character the most obvious feature usually being a change in color. The color of the heartwood differs in different trees, and gives the characteristic appearance to the various kinds of lumber. For example, walnut, oak or cherry lumber is recognized by and valued for the characteristic color of the heartwood of each.  Heat, a form of energy whose presence produces such sensations as those described by the words cold, cool, warm, hot. Two fundamental questions concerning the 