Page:EB1911 - Volume 27.djvu/956

Rh capillary vessels, and these lead into the veins which finally pass back to the heart. The heart is to be regarded as a double organ, each half consisting of an auricle and a ventricle. The right half contains dark venous blood which has been returned from the body and is sent to the lungs: the left heart contains the bright oxygenated blood which has been returned from the lungs and is distributed to the body. There are thus two circulations—the one pulmonary, from the right side of the heart to the pulmonary artery and thence to the capillaries of the lungs and to the left heart by the pulmonary veins—the other systemic, from the left side of the heart, by the aorta, to the arteries and capillaries of the body tissues and organs, whence the blood returns by the veins to the right side of the heart.

A schematic representation is given of the circulatory system in the accompanying diagram. The venous blood flows into the right auricle (RA) from the superior vena cava and the inferior vena cava. The right ventricle (RV) drives through the lungs the blood received from the right auricle. The right auriculo-ventricular valve, or tricuspid, and the pulmonary semi lunar valve are represented directing the flow of blood in this direction. From the pulmonary capillaries the blood returns by the pulmonary veins (PV) into the left auricle (LA), and so through the left auriculo-ventricular or mitral valve

.1.—General Course of Circulation and  some of the Principal Vessels. H′, right ventricle: H, left ventricle; A, A, A, aorta; h, part of left auricle; P, pulmonary artery, going to lungs; P, pulmonary veins; v, ascending or lower vena cava; e, trachea or wind-pipe; p, p′ bronchial tubes; a′, a, right and left carotid arteries; v, v′, veins from root of neck (internal jugular and subclavian), joining to form descending or upper vena cava; i, hepatic artery l, hepatic vein; I, superior mesenteric artery, going to mesentery and bowels; L, portal vein, going to liver; k′, renal artery; k, renal vein; V, inferior vena cava, splitting into the two iliac veins, v, v.

. 2.—Scheme of the Circulation of the Blood in Man, standing erect. The venous system is stippled. C, rigid cranial wall; N, muscles and cutaneous wall of neck; T, thoracic, wall; A, muscular and cutaneous wall of abdomen; D, diaphragm; L, muscles and cutaneous wall of limbs; P, pericardium; AO, aorta; S. V. C, I. V. C, venae cavae; P.V, portal vein; V, valves in veins of neck, or legs; RA, LA, right and left auricles; RV, LV, right and left ventricles.

into the left ventricle (LV). By the left ventricle the blood is driven through the aortic semilunar valve, and is distributed to the systemic arteries, and so to the capillaries of the various organs and back to the veins. The muscular wall of the auricles and that of the right ventricle are much thinner than that of the left ventricle. This is so, because the energy required of the left ventricle must exceed that of the right ventricle, inasmuch as the resistance in the systemic system exceeds that in the pulmonary circuit.

The heart fills with venous blood during its expansion or diastole, and 'forces the blood into the arteries during its contraction or systole. The large arteries are of less capacity than the corresponding veins, and their walls are essentially ex tensile and elastic.' The pulmonary arteries are especially ex tensile structures. The small arteries and arterioles are essentially muscular' tubes and can vary considerably in diameter. The arterioles open into the capillaries, and these are so numerous that each organ may be regarded as a sponge full of blood. The skeletal muscles and the muscular walls of the viscera at each contraction express the blood within them, and materially influence the circulation. The whole muscular system, as well as the heart, must therefore be regarded as a pump to the vascular system. The capillary wall is composed 'of a single layer of flattened cells, separating the blood within from the tissues without. Through this layer, which is of extraordinary tenuity, there takes place an exchange of material between the blood and the tissues, an exchange which depends on the physico-chemical conditions which characterize the living state of the cells. The phenomena of adsorption and osmosis come into play here, but the conditions still await complete elucidation. The veins are of larger calibre= than the corresponding arteries, and have tough and inextensible walls. Their walls are muscular, and contract on local stimulation. Theveins, are not, as 'a-rule, distended with blood to their full potential capacity. The latter is so great that the whole blood of the body can collect within the veins.

The heart and lungs are placed within the thoracic cavity (T), the floor of which is formed by the muscular diaphragm (D); the heart is itself enclosed in a tough inextensible bag, the pericardium (P), the function of which is to check over dilatation of the heart. The pericardium bears to the muscular wall of the heart the same relation as the leather case of a football does to the bag within. In particular, it prevents over-distension of the heart during muscular efforts. The abdominal organs and blood vessels are encompassed by the muscular wall of the abdomen (A), and may be regarded as enclosed in a sphere of muscle. Above is the dome of the diaphragm (T), and below the basin-like levator ani, closing the outlet of the pelvis; in front are the recti muscles, behind the quadrati lumborum and the spine; while the oblique and transverse muscles complete the wall at either side. The brain is enclosed in a rigid and unyielding box of bone—the cranium, while the limbs are encompassed by the ex tensile and, in health, taut and elastic skin.

The heart’s energy is spent in maintaining a pressure of blood in the elastic arteries, and by the difference of pressure in the arteries and veins the blood is kept flowing through the capillaries into the veins. The movements of the body and particularly of respiration help to return the blood from the capillaries and veins back to the heart, valves being set in the veins to direct the blood in this direction. The blood is a viscous fluid and its viscosity varies; it is propelled by a heart which varies both in rate and energy; it circulates through a system of muscular and elastic arteries and veins, which Varies in capacity and may alter in elasticity. The width of bed through which it flows varies greatly at different parts of the circuit, and the resistance offered to the moving blood is very much greater in the capillary-sized vessels than in the large arteries and veins. The blood continually varies, both in quantity and in quality, as it effects exchanges through the capillary walls with the tissues. The problems of the