CHAPTER XI.

THE CIRCULATION. — BLOOD. — LYMPH.

225. The blood, as we have seen, is the principal form which the nutritive constituents of food take after digestion. It flows as pure blood in one set of currents, from the heart to every cell and tissue for their nourishment, and returns in another set of currents to the heart laden with waste products, which are expelled from the body through the lungs and other excretory organs. This flow of the blood to and from the various parts of the body is the circulation, and the organs through which it is propelled are the organs of circulation. These organs are the heart and the blood-vessels, the latter consisting of the arteries, capillaries, and veins.

226. The heart is a hollow, muscular, pear-shaped organ, about the size of the clenched fist. It is situated obliquely in the thoracic cavity, between the two lungs, chiefly on the left side of the body. Its lower pointed end, or apex, strikes against the walls of the thorax, between the fifth and sixth ribs, a little to the left of the breast bone. At this point we can best feel the impulse of the organ.1 The broadened upper end, called the base, is about on a level with the middle of the breast bone, near its junction with the cartilages of the third ribs. Owing

1 The place and extent of the heart's impulse vary a little with the position of the body, and the motions of breathing. The length of the heart is about five inches; its weight in men is about 10 or 12 ounces, in women 8 or 10.

to its surroundings this end of the heart has comparatively little motion. A portion of the right and lower border of the heart rests upon the diaphragm, and is upon the right side of the "median line" of the body.1 The left border is entirely upon the left of this line. (Fig. 47.)

Heart, front view. -1, right ventricle; 2, left ventricle; 3 and 4, right auricle; 5 and 6, left auricle; 7, pulmonary artery; 8, the aorta; 9, superior vena cava; 10 and 11, front coronary artery and vein which in part control the blood-supply of the substance of the heart; 12, lymphatic vessels.

227. The whole organ, with about two inches of the great blood-vessels which arise from it, is enveloped in a fibrous sac known as the pericardium.1 This sac is lined with a smooth, glistening membrane, which secretes a lubricating fluid called serum, thus permitting the heart to move freely and without friction. The interior of the

Representing the cavities of the heart, and the blood-vessels opening into and out of them. The ventricles are separated in the cut to show the commencement of the aorta. The arrows show the direction of the blood-currents.

heart is also lined with a smooth, serous membrane, called the endocardium,2 which is similar to and continuous with the lining membrane of the blood-vessels.

228. The heart is divided by muscular walls into four compartments or cavities, the two upper ones called

1 Derived from the Greek, and signifies" around the heart."
2 Derived from the Greek, and signifies "within the heart."

auricles,1 and the two lower, ventricles.2 The first two have veins which open into them, the last have arteries which arise from them. The auricles receive the blood coming into them through the veins, and when full simultaneously contract and force it into their respective ventricles, through openings (one between each auricle and ventricle), which are known as the auriculo-ventricular openings. The ventricles then simultaneously contract and expel the blood into the arteries. (Fig. 49.)

The openings between the auricles and ventricles, and those between the ventricles and the arteries which connect with them, are guarded by little doors or valves composed of delicate but strong fibrous tissue. These open to allow the blood to pass onward in its natural course, and then close, thus preventing the blood from flowing back, i.e., regurgitating.3

The cavities upon the right side of the heart are called, respectively, the right auricle and right ventricle, and those upon the left side, the left auricle and left ventricle.1 The cavities of the left side of the heart are respectively smaller than those of the right, but their walls are stronger. Especially is this true of the left ventricle, whose function it is to send blood through the entire body. (Fig. 50.)

(Latin), so called, it is said, from their resemblance to a

2 Literally, the diminutive of stomach. The appellation is old, and is used by Cicero.

3 The valves between the auricles and ventricles are operated by slender but powerful muscles within the ventricles. The tendons of these muscles attached to the valves are known as the "chordae tendinae," or tendinous cords. (Fig. 50.) The valves between the left auricle and ventricle are known as the "mitral," from a supposed resemblance, when they are open, to a mitre. Between the right auricle and ventricle are the "tricuspid" valves, i.e., having three points. Between the ventricles and the arteries are the "semilunar" valves, so called from their shape.

4 Sometimes the heart is considered as a double organ, the right side, transmitting venous blood, is spoken of as the right heart, and the left side, transmitting arterial blood, as the left heart.

229. The movements of the blood will probably be best understood if we follow it from point to point in its circuit.

In the first place the venous or impure blood, collected by the smaller veins from the various parts of the body, is poured into two great veins which open into the right auricle. When the auricle is dilated and filled to its normal limit, its walls contract and expel the blood through

The heart and some of its vessels; the ventricles are laid open to show their structure.-A, aorta; PA, pulmonary artery; PV, pulmonary veins of left auricle; L A, left auricle; D V, descending vein, or superior vena cava; A V, ascending vein, or inferior vena cava; L V, left ventricle; R V, right ventricle. The relative thickness of the walls of the ventricles are shown, also the muscle columns and their tendons, together with the curtain-like valves.

the right ventricular opening into the right ventricle. The ventricle thus dilated and filled, contracts, and expels its contents through the pulmonary artery into the lungs, where the blood is thoroughly distributed by numerous capillary" or hair-like blood-vessels among the air cells, and is purified by exchanging its waste products for the

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