 Section I of Grey's Anatomy Part III. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recorded by Laurie Ann Walden. Anatomy of the Human Body, Part III, by Henry Gray. Section I, Angiology, Introduction. The vascular system is divided for descriptive purposes into A, the blood vascular system, which comprises the heart and blood vessels for the circulation of the blood, and B, the lymph vascular system, consisting of lymph glands and lymphatic vessels through which a colorless fluid the lymph circulates. It must be noted, however, that the two systems communicate with each other and are intimately associated developmentally. The heart is the central organ of the blood vascular system and consists of a hollow muscle. By its contraction the blood is pumped to all parts of the body through a complicated series of tubes termed arteries. The arteries undergo enormous ramification in their course throughout the body and end in minute vessels called arterioles, which in their turn open into a close meshed network of microscopic vessels termed capillaries. After the blood has passed through the capillaries it is collected into a series of larger vessels called veins by which it is returned to the heart. The passage of the blood through the heart and blood vessels constitutes what is termed the circulation of the blood, of which the following is an outline. The human heart is divided by septa into right and left halves, and each half is further divided into two cavities, an upper termed the atrium and a lower the ventricle. The heart therefore consists of four chambers, two the right atrium and right ventricle forming the right half, and two the left atrium and left ventricle the left half. The right half of the heart contains venous or impure blood, the left arterial or pure blood. The atria are receiving chambers and the ventricles distributing ones. From the cavity of the left ventricle the pure blood is carried into a large artery, the aorta, through the numerous branches of which it is distributed to all parts of the body with the exception of the lungs. In its passage through the capillaries of the body the blood gives up to the tissues the materials necessary for their growth and nourishment, and at the same time receives from the tissues the waste products resulting from their metabolism. In so doing it is changed from arterial into venous blood which is collected by the veins and through them returned to the right atrium of the heart. From this cavity the impure blood passes into the right ventricle and is then conveyed through the