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membranous bag called the pericardium. The inner lining of the pericardium secretes a fluid in which the heart lies. This fluid prevents any friction which otherwise might arise from the con stant movement of the heart against the surrounding tissues. When, for any reason, the pericardial fluid is not secreted, inflammation arises in that region.

VENACAVA
INFERIOR

A

Internal Structure of Heart. If we should cut open the heart of an ox, down the midline, we could divide it into two parts, each of which would have no internal connection with the other. Each side of the heart is found to be distinct and to be made up of a thin-walled portion with a rather large internal cavity, the auricle, and a smaller portion with heavy muscular walls, the ventricle. The auricles occupy the base of the cone-shaped heart; the ventricles, the apex. The auricle of the right side communicates with the ventricle of that side. In the same manner the auricle of the left side is connected with the ventricle on the left side. Communication between auricles and ventricles is guarded by little flaps of muscle called valves. The auricles receive blood from the veins. The ventricles pump the blood into the arteries. From each ventricle, large arteries leave the heart; that of the left side is called the aorta. pump action of the valves Through the aorta, blood passes to all

Diagrams illustrating the force

of the heart; A, during the filling of the right ventricle;

parts of the body. On the right side, the

B, during the contraction of pulmonary artery carries blood to the

the ventricle.

SUPERIOR

VENACAVA

RIGHT
AURICLE

TRICUSPID
VALVES OPEN

RIGHT
VENTRICLE

TRICUSPID
VALVES
CLOSED

PULMONARY

ARTERY

PULMONARY

VALVES

ULMONARY
OPEN

lungs. The openings to these arteries are guarded by three halfmoon-shaped flaps, which open so as to allow blood to pass away from the ventricle, but not to go back into it when the muscles relax. The heart is constructed on the same plan as a pump, the valves preventing the reflux of blood into the auricle after it is forced out of the ventricle.

The Heart in Action. In a quiet room, the pulsation of the heart may be distinctly heard. A long sound, lub, is followed by a short one, dûb. The first sound is caused by the contraction of the muscles of the heart; the latter sound by the closing of the valves in the heart. The action of the heart is somewhat like that occurring when we squeeze water through a rubber bulb. Blood enters. the auricles from the veins because the muscles of that part of the heart relax; this allows the space within the auricles to fill. Almost immediately the muscles of the ventricles relax, thus allowing blood to pass into the chambers within the ventricles. Then, after a short pause, during which time the muscles of the heart are resting, a wave of muscular contraction begins in the auricles and ends in the ventricles, with a sudden forceful contraction which forces the blood out into the arteries. This contraction of the heart is known as a systole. The extension of the muscles, to allow the auricles and ventricles to fill, is called a diastole. Blood is kept on its course by the valves, which act in the same manner as do the valves in a pump, thus forcing the blood to pass into the arteries upon the contraction of ventricle walls.

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Transverse section of an artery, showing muscular walls.

THE WORK OF THE HEART.—The work performed by the heart is considerable. The two ventricles, at each pulsation, expel about a cup and a half of blood into the arteries of the body. The average rate of the heart beat is about seventy to the minute, so that the work of the ventricles, in a single day, is estimated to release enough energy to lift 193 tons one foot from the ground. The heart is estimated to do as much work in a single day as a moderately heavy man would perform in climbing a mountain 3600 feet in height.

Demonstration. The Circulation of Blood in a Frog's Foot. Bore a half inch hole in one end of a shingle. Wrap a live frog in wet flannel or absorbent cotton, and bind it, by means of elastic bands, upon the board so that the web of the foot is stretched in a horizontal position over the hole. Keep the web of the foot wet. Cover it with a large coverslip. Place it on the stage of a compound microscope, focus with low and then with the high power. A network of blood vessels will be seen which may be partially obscured by numerous pigment cells (dark-colored cells of irregular shape). The blood vessels may easily be recognized by the fluid contents, the ovoid corpuscles floating in the transparent plasma. Note that in some of the blood tubes the blood moves in regular spurts. These are the arteries.

HUNTER'S BIOL.-23

Trace the artery until it breaks out into very tiny tubes, the capillaries. Notice that in the capillaries the diameter of the tube is little more than that of a red corpuscle. Follow the capillaries until they come together to form a vein. What difference in the movement of the blood do you notice in the veins?

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Capillary circulation in the web of a frog's foot, as seen under the compound microscope; a, b, small veins; d, capillaries in which the oval corpuscles are seen to follow one another in single series; c, pigment cells in the skin.

Structure of the Arteries. - A distinct difference in structure exists between the arteries and the veins in the human body. The arteries, because of the greater strain received from the blood which is pumped from the heart, have thicker muscular walls, and, in addition, are very elastic.

CAUSE OF THE PULSE. The pulse, which can easily be detected by pressing the large artery in the wrist or the small one in front of and above the external ear, is caused by the gushing of blood through the arteries after each pulsation of the heart. In the earthworm, we found that certain parts of the blood vessels take up the work of pumping the blood. These vessels which connect the dorsal with the ventral blood vessels are called hearts. Each is a single muscular tube. The fish has such a heart. In the higher vertebrates the heart is more complex, being composed of two such muscu

lar tubes, side by side, each having two chambers (auricle and ventricle). As the large arteries pass away from the heart, the diameter of each individual artery becomes smaller. At the very end of their course, these arteries are so small as to be almost microscopic in size. They are very numerous. There are so many if they were placed together, side by side, their united diameter would be much greater than the diameter of the large artery (aorta) which passes blood from the left side of the heart. This fact is of very great importance, for the force of the blood as it gushes through the arteries becomes very much less when it reaches the smaller vessels. This gushing movement is quite lost when the capillaries are reached. First, because there is so much more space for the blood to fill; secondly, there is considerable friction caused by the very tiny diameter of the capillaries.

Capillary Network

Arteriole

Venule

Capillary network, showing change from arterial to venous blood.

Capillaries. The capillaries form a network of minute tubes everywhere in the body, but especially near the surface and in the lungs. It is through their walls that the food and oxygen pass to the tissues, and carbon dioxide is given up to the plasma. They form the connection that completes the system of circulation of blood in the body.

Function and Structure of the Veins. If the arteries are pipes which supply fluid food to the tissues, then the veins may be likened to drain pipes which carry away waste material from the tissues. Extremely numerous in the extremities and in the muscles and among other tissues of the body, they, like the branches of a

tree, become larger and unite with each other as they approach the heart. The blood supply from the body enters the right heart (auricle) by two large vessels, called, respectively, because of their position, the inferior and superior vena cava.

If the wall of a vein is carefully examined, it will be found to be not so thick or so tough as an artery wall. When empty, a vein collapses; the wall of an artery holds its position. If you hold your hand downward for a little time and then examine it, you will find that the veins, which are relatively much nearer the surface than are the arteries, appear to be very much knotted. This appearance is due to the presence of tiny valves inside the veins. These valves open the direction blood current, but would close if the direction of the blood flow should be reversed (as in case a deep cut severed a vein). As the pressure of blood in the veins is much less than in the arteries, these valves thus aid in keeping the flow of blood in the veins toward the heart.

The Course of the Blood in the Body. Although the two sides of the heart are separate and distinct from each other, yet every drop of blood that passes through the left heart likewise passes through the right heart. There are two distinct systems of circulation in the body. The pulmonary circulation takes the blood through the right auricle and ventricle, to the lungs, and passes it back to the left auricle. This is a relatively short circulation, the blood receiving in the lungs its supply of oxygen, and there giving up some of its carbon dioxide. The greater circulation is known as the systemic circulation; in this system, the blood leaves the left ventricle through the great dorsal aorta. A large part of the blood passes directly to the muscles; some of it goes to the nervous system, kidneys, skin, and other organs of the body. It gives up its supply of food and oxygen in these tissues, receives the waste products of oxidation while passing through the capillaries, and returns to the right auricle through the venæ cavæ.

It

Valves in a vein.

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