nutrients are absorbed by the blood vessels in the walls of the alimentary canal. Since, however, many tissues of the body are at a considerable distance from the organs of digestion, it is evident that some means must be provided for supplying each cell with the nutrients it needs. This is effected by the circulation of the blood. By the term circulation of the blood is meant the ceaseless movement of the blood through a system of tubes called blood vessels.

-

154. Organs of circulation. As is also true in the fish and other vertebrates, the force that drives the blood around through the body is largely furnished by the contraction of the muscular walls of the heart. Any blood vessel that carries blood away from the heart is called an artery.1 The veins are the blood vessels that bring the blood back to the heart. Connecting the arteries and the veins in every part of the body are countless microscopic blood vessels called capillaries (Latin, capillus hair, so called from their minute size). We shall now consider in more detail the structure and action of each of these circulatory organs.

III. THE HEART

155. Position, size, shape. The heart (Fig. 2) is a conical or pear-shaped organ about the size of the fist. It lies behind the breastbone near the middle of the chest cavity, with its pointed end or apex extending toward the left side between the fifth and sixth ribs. Since the beat of the heart is felt most plainly near the apex, it is commonly but wrongly believed that the heart lies on the left side of the body. Let one imagine the front wall of the chest

1 From Greek, aer = airterein = to hold a name which was given by the early anatomists to these tubes, because they were found empty after death, and were therefore supposed to carry air.

cavity to be removed; one would then see the soft, pink lungs on either side, nearly filling the chest cavity, and between them the heart 1 (Fig. 2).

1

We have seen (A. B., 99)

156. Chambers of the heart. that a fish's heart has two chambers, an auricle to receive the blood from all parts of the body, and a muscular ventricle to force the blood into the arteries which carry it to the organs of respiration (gills) and thence by another system of arteries to all parts of the fish's body. In the human circulatory system, the blood, after returning to the heart from the organs of the body, is likewise forced through an auricle, a ventricle, and arteries, and so reaches the breathing organs (lungs). Unlike the circulation in the fish, however, the blood does not pass from the breathing organs to the other parts of the body directly, but returns by veins to the heart, and so another auricle and ventricle are provided on the left side of the heart. These receive the blood from the organs of respiration, and force it to all parts of the body. Thus we see that we have two hearts, the chambers of which are completely separated by a muscular partition; the right heart receiving the blood from all over the body and pumping it to the lungs; the left heart receiving the blood from the lungs and pumping it over all the body.

A comparison of these four chambers shows important differences. In the first place, the auricles have relatively thin walls as compared with the ventricles, and the reason for this is evident when we see that their function is simply to receive the blood from the veins and to push it downward into the ventricles. When one compares the walls of the

1 The heart is not only surrounded by the skeleton and muscles of the chest wall, but it is also inclosed in a tough bag of connective tissue called the pericardium (Greek, peri = around cardia = heart).

left ventricle with those of the right, one is struck with the great thickness of the former. The left ventricle does much more work than the right; it forces blood to the top of the head, to the tips of the fingers and toes, and to every other organ of the body. The right ventricle, on the other hand, pumps blood only to the lungs (Fig. 33).

157. Action of the heart.

The blood flows into the right and left auricles and thence into the corresponding ventricles. When the ventricles are nearly full of blood, the two auricles contract and force downward enough blood to fill the two ventricles completely. These muscular chambers then contract and force the blood out into the arteries that lead to the lungs, or to other parts of the body. When the contraction of the ventricles takes place, it is evident that blood I would be driven back into the auricles were there not some means of preventing this back flow. Hence, between each

auricle and ventricle tough flaps of membrane are provided which close the opening while the ventricles are contracting. Connected with each of these flaps are tough cords of tissue that are attached to the muscular walls of the ventricle. These cords prevent the valves from being forced up into the

A

positions of valves before

the contraction of the ven-
tricle.

B = position of valves at the
beginning of the contrac-
tion of the ventricle.

FIG. 34.-Diagrams to show the action of the valves of the heart.

auricle (Fig. 34). When the ventricles cease to contract, the blood entering the auricles presses these valves downward and so enters the ventricles.

IV. THE BLOOD VESSELS

158. Position of arteries and the pulse. We have defined an artery as a blood vessel carrying blood from the heart. Every time the ventricles contract, the arteries leading from them are expanded, and this is true of every artery in the body. Most arteries lie beneath thick layers of muscle or bone, which protect them from possible injury; but in certain regions of the body they lie close to the surface. If one places the fingers on the wrist two inches or more below the ball of the thumb, it is possible to feel a

distinct throbbing, called the pulse. This is due to the enlargement of the artery at each heart beat followed by subsequent contraction. When an artery is cut, therefore, the blood is forced out in spurts at each contraction of the ventricle.

159. Structure of arteries.

If a piece of the aorta of any animal is examined, it will be found that the blood vessel retains its tubular form, and this is due to the presence

of thick layers of muscular and elastic tissue (Fig. 35). It is the elastic tissue that allows the arteries to expand when more blood is forced into them by the contraction of the ventricles. After each pulse these elastic walls squeeze the blood forward into the capillaries; arteries, therefore, are specially adapted to keep the capillaries full of blood.

The muscular tissue in the walls of the arteries aids in regulating the size of the arteries, and so determines the relative amount of blood supplied to any given organ. For example, when the face is flushed, the muscles in the arteries have relaxed; pallor, on the other hand, is due to the contraction of the muscular walls.