*Valves of the Heart.*-Located at suitable places in the heart are four gate-like contrivances, called valves. The purpose of these is _to give the blood a definite direction_ in its movements. They consist of tough, inelastic sheets of connective tissue, and are so placed that pressure on one side causes them to come together and shut up the pa.s.sageway, while pressure on the opposite side causes them to open. A valve is found at the opening of each auricle into the ventricle, and at the opening of each ventricle into the artery with which it is connected.

The valve between the right auricle and the right ventricle is called the _tricuspid_ valve. It is suspended from a thin ring of connective tissue which surrounds the opening, and its free margins extend into the ventricle (Fig. 16). It consists of three parts, as its name implies, which are thrown together in closing the opening. Joined to the free edges of this valve are many small, tendinous cords which connect at their lower ends with muscular pillars in the walls of the ventricle. These are known as the _chordae tendineae_, or heart tendons. Their purpose is to serve as _valve stops_, to prevent the valve from being thrown, by the force of the blood stream, back into the auricle.

The _mitral_, or bicuspid, valve is suspended around the opening between the left auricle and the left ventricle, with the free margins extending into the ventricle. It is exactly similar in structure and arrangement to the tricuspid valve, except that it is stronger and is composed of two parts instead of three.

[Fig. 16]

Fig. 16-*Right side of heart* dissected to show cavities and valves. _B._ Right semilunar valve. The tricuspid valve and the chordae tendineae shown in the ventricle.

The _right semilunar_ valve is situated around the opening of the right ventricle into the pulmonary artery. It consists of three pocket-shaped strips of connective tissue which hang loosely from the walls when there is no pressure from above; but upon receiving pressure, the pockets fill and project into the opening, closing it completely (Fig. 16). The _left semilunar_ valve is around the opening of the left ventricle into the aorta, and is similar in all respects to the right semilunar valve.

*Differences in the Parts of the Heart.*-Marked differences are found in the walls surrounding the different cavities of the heart. The walls of the ventricles are much thicker and stronger than those of the auricles, while the walls of the left ventricle are two or three times thicker than those of the right. A less marked but similar difference exists between the auricles and also between the valves on the two sides of the heart.

These differences in structure are all accounted for by the work done by the different portions of the heart. The greater the work, the heavier the structures that perform the work.

[Fig. 17]

Fig. 17-*Diagram of the circulation*, showing in general the work done by each part of the heart. The right ventricle forces the blood through the lungs and into the left auricle. The left ventricle forces blood through all parts of the body and back to the auricle. The auricles force blood into the ventricles.

*Connection with Arteries and Veins.*-Though the heart is in communication with all parts of the circulatory system, it makes actual connection with only a few of the blood tubes. These enter the heart at its upper portion (Fig. 15), but connect with its different cavities as follows:

1. _With the right auricle_, the superior and the inferior venae cavae and the coronary veins. The superior vena cava receives blood from the head and the upper extremities; the inferior vena cava, from the trunk and the lower extremities; and the coronary veins, from the heart itself.

2. _With the left auricle_, the four pulmonary veins. These receive blood from the lungs and empty it into the left auricle.

3. _With the right ventricle_, the pulmonary artery. This receives blood from the heart and by its branches distributes it to all parts of the lungs.

4. _With the left ventricle_, the aorta. The aorta receives blood from the heart and through its branches delivers it to all parts of the body.

*How the Heart does its Work.*-The heart is a muscular pump(18) and does its work through the contracting and relaxing of its walls. During contraction the cavities are closed and the blood is forced out of them.

During relaxation the cavities open and are refilled. The valves direct the flow of the blood, being so arranged as to keep it moving always in the same direction (Fig. 17).

The heart, however, is not a single or a simple pump, but consists in reality of _four_ pumps which correspond to its different cavities. These connect with each other and with the blood vessels over the body in such a manner that each aids in the general movement of the blood.

[Fig. 18]

Fig. 18-Diagram ill.u.s.trating the "cardiac cycle."

*Work of Auricles and Ventricles Compared.*-In the work of the heart the two auricles contract at the same time-their contraction being followed immediately by the contraction of both ventricles. After the contraction of the ventricles comes a period of rest, or relaxation, about equal in time to the period of contraction of both the auricles and the ventricles.(19) On account of the work which they perform, the auricles have been called the "feed pumps" of the heart; and the ventricles, the "force pumps."(20) It is the function of the auricles to collect the blood from the veins, to let this run slowly into the ventricles when both the auricles and ventricles are relaxed, and finally, by contracting, _to force an excess of blood into the ventricles_, thereby distending their walls. The ventricles, having in this way been fully charged by the auricles, now contract and force their contents into the large arteries.

*Sounds of the Heart.*-Two distinct sounds are given out by the heart as it pumps the blood. One of them is a dull and rather heavy sound, while the other is a short, sharp sound. The short sound follows quickly after the dull sound and the two are fairly imitated by the words "lubb, dup."

While the cause of the first sound is not fully understood, most authorities believe it to be due to the contraction of the heart muscle and the sudden tension on the valve flaps. The second sound is due to the closing of the semilunar valves. These sounds are easily heard by placing an ear against the chest wall. They are of great value to the physician in determining the condition of the heart.

*Arteries and Veins.*-These form two systems of tubes which reach from the heart to all parts of the body. The arteries receive blood from the heart and distribute it to the capillaries. The veins receive the blood from the capillaries and return it to the heart. The arteries and veins are similar in structure, both having the form of tubes and both having three distinct layers, or coats, in their walls. The corresponding coats in the arteries and veins are made up of similar materials, as follows:

1. _The inner coat_ consists of a delicate lining of flat cells resting upon a thin layer of connective tissue. The inner coat is continuous with the lining of the heart and provides a smooth surface over which the blood glides with little friction.

2. _The middle coat_ consists mainly of non-striated, or involuntary, muscular fibers. This coat is quite thin in the veins, but in the arteries it is rather thick and strong.

3. _The outer coat_ is made up of a variety of connective tissue and is also much thicker and stronger in the arteries than in the veins.

[Fig. 19]

Fig. 19-Artery dissected to show the coats.

Marked differences exist between the arteries and the veins, and these vessels are readily distinguished from each other. The walls of the arteries are much thicker and heavier than those of the veins (Fig. 19).

As a result these tubes stand open when empty, whereas the veins collapse.

The arteries also are highly elastic, while the veins are but slightly elastic. On the other hand, many of the veins contain valves, formed by folds in the inner coat (Fig. 20), while the arteries have no valves. The blood flows more rapidly through the arteries than through the veins, the difference being due to the fact that the system of veins has a greater capacity than the system of arteries.

[Fig. 20]

Fig. 20-Vein split open to show the valves.

*Why the Arteries are Elastic.*-The elasticity of the arteries serves a twofold purpose. It keeps the arteries from bursting when the blood is forced into them from the ventricles, and it is a means of _supplying pressure to the blood while the ventricles are in a condition of relaxation._ The latter purpose is accomplished as follows:

Contraction of the ventricles fills the arteries overfull, causing them to swell out and make room for the excess of blood. Then while the ventricles are resting and filling, the stretched arteries press upon the blood to keep it flowing into the capillaries. In this way _they cause the intermittent flow from, the heart to become a steady stream in the capillaries_.

The swelling of the arteries at each contraction of the ventricle is easily felt at certain places in the body, such as the wrist. This expansion, known as the "pulse," is the chief means employed by the physician in determining the force and rapidity of the heart's action.

*Purpose of the Valves in the Veins.*-The valves in the veins are not used for directing the _general_ flow of the blood, the valves of the heart being sufficient for this purpose. Their presence is necessary because of the pressure to which the veins are subjected in different parts of the body. The contraction of a muscle will, for example, close the small veins in its vicinity and diminish the capacity of the larger ones. The natural tendency of such pressure is to empty the veins in two directions-one in the same direction as the regular movement of the blood, but the other in the opposite direction. The valves by closing cause the contracting muscle to push the blood in one direction only-toward the heart. The valves in the veins are, therefore, an economical device for _enabling variable pressure_ in different parts of the body _to a.s.sist in the circulation_.

Veins like the inferior vena cava and the veins of the brain, which are not compressed by movements of the body, do not have valves.

*Purposes of the Muscular Coat.*-The muscular coat, which is thicker in the arteries than in the veins and is more marked in small arteries than in large ones, serves two important purposes. In the first place it, together with the elastic tissue, keeps the capacity of the blood vessels _equal to the volume of the blood_. Since the blood vessels are capable of holding more blood than may be present at a given time in the body, there is a liability of empty s.p.a.ces occurring in these tubes. Such s.p.a.ces would seriously interfere with the circulation, since the heart pressure could not then reach all parts of the blood stream. This is prevented by the contracted state, or "tone," of the blood vessels, due to the muscular coat.

In the second place, the muscular coat serves the purpose of _regulating_ the amount of blood which any given organ or part of the body receives.

This it does by varying the caliber of the arteries going to the organ in question. To increase the blood supply, the muscular coat relaxes. The arteries are then dilated by the blood pressure from within so as to let through a larger quant.i.ty of blood. To diminish the supply, the muscle contracts, making the caliber of the arteries less, so that less blood can flow to this part of the body. Since the need of organs for blood varies with their activity, the muscular coat serves in this way a very necessary purpose.

[Fig. 21]

Fig. 21-Diagram of network of capillaries between a very small artery and a very small vein. Shading indicates the change of color of the blood as it pa.s.ses through the capillaries. _S._ Places between capillaries occupied by the cells.

*Capillaries.*-The capillaries consist of a network of minute blood vessels which connect the terminations of the smallest arteries with the beginnings of the smallest veins (Fig. 21). They have an average diameter of less than one two-thousandth of an inch (12 ) and an average length of less than one twenty-fifth of an inch (1 millimeter). Their walls consist of a single coat which is continuous with the lining of the arteries and veins. This coat is formed of a single layer of thin, flat cells placed edge to edge (Fig. 22). With a few exceptions, the capillaries are found in great abundance in all parts of the body.

[Fig. 22]

Fig. 22-*Surface of capillary* highly magnified, showing its coat of thin cells placed edge to edge.