Now that we know something of the blood-vessels, which carry the blood, we naturally turn to the central pump which forces it along.
The heart is situated in the middle of the thorax, or chest, between the two lungs. It is a pear- shaped organ, and has its broad end or base turned upward andbackward towards the spine, and its pointed end or apex downward, forward, and to the left.
It is a hollow organ, about the size of one"s closed fist, and ismade entirely of muscle. It contains two great chambers, completely separated from each other by an immovable wall of muscle, which reaches from the base to the apex. Each of these great chambers is further subdivided by another partition, stretching cross-wise. Thus, the heartreally contains four chambers of the same size-twoupper and two lower.
The upper chambers are called the right and leftauricles; the lower, the right and left ventricles. There is a passage between the upper and lower chamber on each side, so that whatever is in the auricle can pass through into the ventricle below, but there is no direct communication between the right and left sides. The heart is a muscular organ, and its pumping work is accomplished by the contraction of its muscles. These muscles, under the direct guidance and control of certain nerves, always work in one particular way. The auricles and ventricles are never at work together.
Both auricles begin to contract at the same moment, but this is only the signal for the ventricles to cease working, and when the ventricles in their turn begin to contract, the auricles cease work. In this way, at the very moment when the two ventricles are contracted to their smallest size, the two auricles are expanded or stretched out to their fullest extent. When the auricles take their turn to work or contract, the ventricles in like manner expand to their utmost.
It is by this alternate contraction of auricles and ventricles that the heart does its pumping. Every time the auricles contract they force the blood that is in them through the passage in their floor into the ventricles below; every time the ventricles contract, the blood is driven out from them in like manner. We want to see what becomes of it. Let us begin with the blood in the auricles, for we have not yet learned how it gets there.
The veins from all parts of the body collect up the impure, purple blood, and unite again and again into larger trunks on their way to the heart. They finally pour their contents into the expanded right auricle of the heart by two great vessels-one draining, so to speak, the upperpart of the body, the other, the lower. These two greatveins are called respectively the superior and the inferior venacava.
Once in the right auricle, the blood is forced by the next contraction into the right ventricle, but the ventricle is no sooner filled than it begins in turn to contract, and this must force the blood somewhere. It cannot flow back into the auricle, because the passage between the two chambers is guarded by a valve which blocks the way. There is an easy passage, however, by which the blood can leave the ventricle, and that is the entrance to a great vessel, called the pulmonary artery. When the contraction of the ventricle comes, therefore, the blood takes the only course open and rushes into this great artery. It is called the pulmonary artery because it leads to the lungs. Remember that the blood which it carries away from the right ventricle of the heart to the lungs is the same impure, purple blood which the great veins poured into the auricle. Both chambers on the right side of the heart contain this dark purple blood.
The blood once in the great artery must not be suffered to return; it must go forward. This is provided for by apouch-like valve, known as the semi-lunar valve, which guards the entrance. It consists of three little pouches with their mouths all opening towards the artery, and away from the heart. They allow an easy passage onward, but the least attempt of the blood to flow back would fillthose little bags and so block the way.
It is curious to note that the muscular walls of this right ventricle are thicker and stronger than those of the auricle above it. The auricle has merely to squeeze the blood into the ventricle; the ventricle has to pump it out of the heart altogether into the lungs.
If we next turn our attention to the left side of the heart, commencing with the auricle on that side, we shall find four large vessels opening into it. These are the pulmonary veins, so called because they come from the lungs-two from each lung. They bring back to the heartthe blood which was sent to the lungs, but it comes backno longer impure and purple, but pure and bright scarlet in color.
The expanding auricle receives it from these vessels, and as soon as it is full begins to contract, the contraction forcing it through into the ventricle below. The passage between the two chambers is guarded, like that on the right side, by a valve. Hence when the ventricles contract the blood cannot return.
Leading from this left ventricle is a great artery- the main artery for supplying blood to the body. It iscalled the aorta, and its entrance is guarded, like that of the pulmonary artery on the other side, with a valve to prevent any return, but to allow an easy passage onward. The contraction of the ventricle forces the blood along this channel, and the work of the heart is complete.
Semi-lunar valves, like those at the entrance of the pulmonary artery, guard the gateway into the aorta, and the enormously increased labor of pumping the blood into every corner of the body is also provided for. The walls of the left ventricle are much thicker and strongerthan those of any other part of the organ.