There are in reality two circulations, the pulmonary and the systemic. The dark, venous blood returned from the circuit through the body, the systemic circulation, where it has be come charged with waste products and other impurities is poured into the right auricle. This contracts, the tricuspid valve opens and the blood is discharged into the right ventricle. Here it remains a small fraction of a second, when this ventricle contracts, the tricuspid valve closes and the blood is driven through the pul monary artery into the lungs where it is spread out through the millions of capillaries distrib uted throughout the lung tissue. Here it is freed from its carbon dioxide, oxygenated and then returned through the pulmonary vein into the left auricle. The contraction of the left auricle discharges this oxygenated blood through the mitral opening into the left ventricle, whose powerful contraction sends it into the aorta to perform its systemic circuit, which distributes it to the arteries supplying all parts of the body by their continuation through the arterioles which end in turn in the capillaries distributed throughout all the tissues. Here the blood through the thin capillary walls comes into con tact with the intermediary fluid known as the lymph, which fills all the tissue spaces through which by osmosis an interchange takes place. The blood exchanges its oxygen and nutritive elements for the waste matter of the body and effects that transference of chemical substances from the organs in which they are produced to those others in any way depending on their activity. This includes the transmission not only of the elaborated products of digestion but likewise the internal secretions or hormones, the importance of which is coming to recog nition. From the capillaries, after exchange of products with the tissues through the lymph has taken place, the blood is collected by venules into the veins, which finally unite into the vena cava which carries the blood back to the right auricle. The contraction of both sides of the heart is a simultaneous one. It consists in reality of a wave of contractions passing over the heart muscle, the period of contraction be ing known as the systole, while the intervening period of rest and relaxation is known as the diastole. The complete cycle of contraction and relaxation with the circulation of the blood through the body occupies about 22 seconds, and occurs in the adult normally about 72 times - in a minute. The audible sound on examination is due to the closing of the auriculo-ventricular valves and those at the entrance of the aorta and pulmonary artery. The pulse which can
be felt in various superficial localities is due to the expansion of the distensible arterial walls. The elasticity of the arterial walls is an important factor in regulating the original force of the blood flow. Their distension changes the intermittent propulsive flow into a more steady current, while their ready contractility after distension regulates the distribution of the blood stream to the various organs. Pressure is gradually diminished until in the veins there is merely a uniform flow of the returning blood. The contraction of the heart muscles is due to a series of complicated nerv ous mechanisms. The heart musculature mani fests in its arrangement its development from an original muscular tube which, while it has been modified into the separate chambers of the mammalian heart, has yet •retained its continuity in the band of muscular fibres dis covered by His (auriculo-ventricular bundle) which allows the contraction impulse to proceed successively from the root of this bundle in the right auricle to the ventricles. The excitation of the heart muscle Seems to be due to the molecular concentration of the fluid which bathes the tissue. The heart is also under the control of a special innervation which adapts it to conditions of the organism and to changes in the arterial resistance. The vagus nerve from the medulla oblongata exercises an in hibitory function while sympathetic fibres run ning from the upper dorsal region of the spinal cord exercise a stimulatory function. Sensa tions arising in the heart are carried through the sympathetic ganglia and the upper thoracic region of the spinal cord where they are re ferred to the sensory nerves of adjacent regions with which they come into relation, so that cardiac pain is not recognized as such but as sensation in these regions. The walls of the blood vessels are also richly supplied with nerves which regulate their tonicity and thus adapt them to the changing conditions of the body organism. These are the vasomotor nerves and consist of the vasoconstrictors which exert a tonic effect and bring about a contrac tion of the vessels and the vasodilator nerves, which as the name implies dilate the vessels. The heart has its special system of blood supply in the coronary vessels, the blood entering the coronary arteries from the aorta and passing into capillaries which surround all the fibres.
See AwrinuEs; BLoon; BLOOD PRESSURE ; CAPIL