The Organs of Circulation.
THE food having been digested, changed to chyle, absorbed by the
lacteals, carried to the veins, poured into the right heart, sent up to
the lungs, and prepared for nourishing the body, will still be useless,
if not distributed to every part of the system. The organs for effecting
this distribution are the heart, the arteries, the veins, and the
capillaries. The Heart is placed obliquely in the chest, with one lung
on each side, and is enclosed between the two folds of the mediastinum
Its form is something like a cone. Its base is turned upward and
backward in the direction of the right shoulder; the apex forward and to
the left, occupying the space between the fifth and sixth ribs, about
three inches from the breastbone. It is surrounded by a membranous case
or sac, called the pericardium. The heart is a muscular body, and has
its fibers so interwoven that it is endowed with great strength. It is a
double organ, having two sides, a right and a left, which are divided
from each other by a muscular partition, called a septum. The right
heart sends the blood to the lungs ; the left heart distributes it to
the general system. Each side is divided into two compartments, an
auricle and a ventricle. The Auricles have thinner walls than the
ventricles, being only reservoirs to hold the blood until the ventricles
force it along to other parts. The Ventricles have within them fleshy
columns, called columnceearne6e. The walls of the left ventricle are
thicker than those of the right, being required to contract with more
force. Each of the four cavities will contain from one and a half to two
ounces of blood.
The Tricuspid valves are situated between the auricle and ventricle on the right side, and consist of three folds of a thin, triangular membrane. The mitral valves occupy the same position on the left side. Small white cords, called chordw tendin6e, pass from the floating edge of these to the columnoe carneae, to prevent the backward pressure of the blood from carrying the valves into the auricles.
The pulmonary artery is the outlet of the right ventricle; the larger artery, called aorta, of the left ventricle. At the opening of these arteries are membranous folds, called semilunar valves. Fig. 33 gives a fine view of the heart: 1, is the right auricle; 2, the left auricle; 3, the right ventricle ; 4, the left ventricle; 5, 6, 7, 8,9,10 the vessels which bring the blood to and carry it away from the heart.
The Arteries are the round tubes which carry the red blood from the left side of the heart to every part of the body. The sides of arteries are stiff and hard, and do not fall together when empty. They may often be seen open in a piece of boiled beef. The arteries have three coats, an external, which is cellular, firm and strong; a middle, which is fibrous and elastic; and an internal, which is serous and smooth, being a continuation of the lining of the heart. They are surrounded by a cell vestment called a sheath, which separates them from surrounding organs.
The Pulmonary Artery starts from the right ventricle in front of the opening of the aorta, and ascends to the under surface of the aortic arch, where it parts into two branches, sending one to the right, the other to the left lung. Having divided and subdivided to a great extent, they end in the capillary vessels, uniting, joining their mouths, and becoming continuous with the pulmonary veins just where they pass around the aircells. The Aorta is the largest artery in the body. It takes a slight turn in the chest, called the arch of the aorta, from which are given off the arteries which carry the blood to the head, etc.; thence it descends into the belly along the side of the backbone, and at the bottom of the abdomen it divides into two arteries, called the iliaes one going to each of the lower limbs. The branches the aorta gives off a supply of red blood to every part of the body. The Veins carry the dark or purple blood. Being made red and vital by meeting atmospheric air in the lungs, and then conveyed to every part of the body in the arteries, the blood loses its redness in the capillaries, and comes back to the heart in the veins, dark and purple, and unfit to support life. The veins are more numerous and nearer the surface than the arteries. They have, likewise, thinner walls, and when empty, they collapse or fall together. They begin in the small capillaries, and running together, they grow larger and larger, and finally form the great trunks which pour the dark blood into the right auricle. The veins are composed of three coats, similar to those of the arteries, with the exception of being thinner and more delicate. These vessels have valves all along their inner surface, to aid in circulating the blood. The large vein which receives all the dark blood from above, and pours it into the right auricle, is called the venaeava descendens ; the one which takes it from below, and disposes of it in the same manner,is the vena eava aseendens. The pulmonary veins bring the red blood from the lungs to the leftauricle, and thus are exceptional in their use, being the only veins which carry red blood.
The Capillaries are the extremely fine network of vessels between the ends of the arteries on the one side, and of the veins on the other. They inosculate, or join their mouths to the very small arteries at one end, and to the equally small veins at the other. They are the industrious little builders of the human frame. Receiving the blood, red, and full of life, from the terminal extremities of the arteries, they take the living particles out of it, and apply them to the renewing and vitalizing of the body, and then pass it along into the hairlike beginnings of the veins, dark and bereft of vitality, to be carried up for another freight of chyle, and to be again vitalized by being touched in the lungs by the breath of heaven.
In Fig. 34 we have a good ideal illustration of the whole circulation. From the right ventricle of the heart, 2, the dark blood is thrown into the pulmonary artery, 3, and its branches, 4, 4, carry it to both lungs. In the capillary vessels, 6, 6, the blood comes in contact with the air, and becomes red and vitalized. Thence it is returned to the left auricle of the heart, 9, by the veins, 7, 8.
Thence it passes into the left ventricle, 10. A forcible contraction of this sends it forward into the aorta, 11. Its branches, 12, 13, 13, distribute it to all parts of the body. The arteries terminate in the capillaries, 14, 14. Here the blood loses its redness, and goes back to the right auricle, 1, by the vena cava descendens, 15, and the vena eava ascendens, 16. The tricuspid valves, 17, prevent the reflow of the blood from the right ventricle to the right auricle. The semilunar valves, 18, prevent the blood from passing back from the pulmonary artery to the right ventricle. The mitral valves, 19, prevent its being forced back from the left ventricle to the left auricle. The semilunar valves, 20, prevent the backward flow from the aorta to the left ventricle. By a careful examination of this diagram, with these explanations, the reader may understand the circulation very well. The passage of the blood from the right heart, through the lungs, and back to the left heart, is called the lesser, or pulmonic circulation ; its passage from the left heart through all parts of the body, and back to the right heart, is the greater or systematic circulation.
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