183. General Plan of Circulation. All the tissues of the body depend upon the blood for their nourishment. It is evident then that this vital fluid must be continually renewed, else it would speedily lose all of its life-giving material. Some provision, then, is necessary not only to have the blood renewed in quant.i.ty and quality, but also to enable it to carry away impurities.

So we must have an apparatus of circulation. We need first a central pump from which branch off large pipes, which divide into smaller and smaller branches until they reach the remotest tissues. Through these pipes the blood must be pumped and distributed to the whole body. Then we must have a set of return pipes by which the blood, after it has carried nourishment to the tissues, and received waste matters from them, shall be brought back to the central pumping station, to be used again. We must have also some apparatus to purify the blood from the waste matter it has collected.

[Ill.u.s.tration: Fig. 68.--Anterior View of the Heart.

A, superior vena cava; B, right auricle; C, right ventricle; D, left ventricle; E, left auricle; F, pulmonary vein; H, pulmonary artery; K, aorta; L, right subclavian artery; M, right common carotid artery; N, left common carotid artery.

This central pump is the heart. The pipes leading from it and gradually growing smaller and smaller are the arteries. The very minute vessels into which they are at last subdivided are capillaries. The pipes which convey the blood back to the heart are the veins. Thus, the arteries end in the tissues in fine, hair-like vessels, the capillaries; and the veins begin in the tissues in exceedingly small tubes,--the capillaries. Of course, there can be no break in the continuity between the arteries and the vein. The apparatus of circulation is thus formed by the heart, the arteries, the capillaries, and the veins.

184. The Heart. The heart is a pear-shaped, muscular organ roughly estimated as about the size of the persons closed fist. It lies in the chest behind the breastbone, and is, lodged between the lobes of the lungs, which partly cover it. In shape the heart resembles a cone, the base of which is directed upwards, a little backwards, and to the right side, while the apex is pointed downwards, forwards, and to the left side.

During life, the apex of the heart beats against the chest wall in the s.p.a.ce between the fifth and sixth ribs, and about an inch and a half to the left of the middle line of the body. The beating of the heart can be readily felt, heard, and often seen moving the chest wall as it strikes against it.

[Ill.u.s.tration: Fig. 69.--Diagram ill.u.s.trating the Structure of a Serous Membrane.

A, the viscus, or organ, enveloped by serous membrane; B, layer of membrane lining cavity; C, membrane reflected to envelop viscus; D, outer layer of viscus, with blood-vessels at E communicating with the general circulation.

The heart does not hang free in the chest, but is suspended and kept in position to some extent by the great vessels connected with it. It is enclosed in a bell-shaped covering called the pericardium. This is really double, with two layers, one over another. The inner or serous layer covers the external surface of the heart, and is reflected back upon itself in order to form, like all membranes of this kind, a sac without an opening.[32] The heart is thus covered by the pericardial sac, but is not contained inside its cavity. The s.p.a.ce between the two membranes is filled with serous fluid. This fluid permits the heart and the pericardium to glide upon one another with the least possible amount of friction.[33]

The heart is a hollow organ, but the cavity is divided into two parts by a muscular part.i.tion forming a left and a right side, between which there is no communication. These two cavities are each divided by a horizontal part.i.tion into an upper and a lower chamber. These part.i.tions, however, include a set of valves which open like folding doors between the two rooms. If these doors are closed there are two separate rooms, but if open there is practically only one room. The heart thus has four chambers, two on each side. The two upper chambers are called auricles from their supposed resemblance to the ear. The two lower chambers are called ventricles, and their walls form the chief portion of the muscular substance of the organ. There are, therefore, the right and left auricles, with their thin, soft walls, and the right and left ventricles, with their thick and strong walls.

185. The Valves of the Heart. The heart is a valvular pump, which works on mechanical principles, the motive power being supplied by the contraction of its muscular fibers. Regarding the heart as a pump, its valves a.s.sume great importance. They consist of thin, but strong, triangular folds of tough membrane which hang down from the edges of the pa.s.sages into the ventricles. They may be compared to swinging curtains which, by opening only one way, allow the blood to flow from the auricles to the ventricles, but by instantly folding back prevent its return.

[Ill.u.s.tration: Fig. 70.--Lateral Section of the Right Chest. (Showing the relative position of the heart and its great vessels, the sophagus and trachea.)

A, inferior constrictor muscle (aids in conveying food down the sophagus); B, sophagus; C, section of the right bronchus; D, two right pulmonary veins; E, great azygos vein crossing sophagus and right bronchus to empty into the superior vena cava; F, thoracic duct; H, thoracic aorta; K, lower portion of sophagus pa.s.sing through the diaphragm; L, diaphragm as it appears in sectional view, enveloping the heart; M, inferior vena cava pa.s.sing through diaphragm and emptying into auricle; N, right auricle; O, section of right branch of the pulmonary artery; P, aorta; R, superior vena cava; S, trachea.

The valve on the right side is called the tricuspid, because it consists of three little folds which fall over the opening and close it, being kept from falling too far by a number of slender threads called chordae tendinae. The valve on the left side, called the mitral, from its fancied resemblance to a bishop's mitre, consists of two folds which close together as do those of the tricuspid valve.

The slender cords which regulate the valves are only just long enough to allow the folds to close together, and no force of the blood pushing against the valves can send them farther back, as the cords will not stretch The harder the blood in the ventricles pushes back against the valves, the tighter the cords become and the closer the folds are brought together, until the way is completely closed.

From the right ventricle a large vessel called the pulmonary artery pa.s.ses to the lungs, and from the left ventricle a large vessel called the aorta arches out to the general circulation of the body. The openings from the ventricles into these vessels are guarded by the semilunar valves. Each valve has three folds, each half-moon-shaped, hence the name semilunar. These valves, when shut, prevent any backward flow of the blood on the right side between the pulmonary artery and the right ventricle, and on the left side between the aorta and the left ventricle.

[Ill.u.s.tration: Fig. 71.--Right Cavities of the Heart.

A, aorta; B, superior vena cava; C, C, right pulmonary veins; D, inferior vena cava; E, section of coronary vein; F, right ventricular cavity; H, posterior curtain of the tricuspid valve; K, right auricular cavity; M, fossa ovalis, oval depression, part.i.tion between the auricles formed after birth.

186. General Plan of the Blood-vessels Connected with the Heart.

There are numerous blood-vessels connected with the heart, the relative position and the use of which must be understood. The two largest veins in the body, the superior vena cava and the inferior vena cava, open into the right auricle. These two veins bring venous blood from all parts of the body, and pour it into the right auricle, whence it pa.s.ses into the right ventricle.

From the right ventricle arises one large vessel, the pulmonary artery, which soon divides into two branches of nearly equal size, one for the right lung, the other for the left. Each branch, having reached its lung, divides and subdivides again and again, until it ends in hair-like capillaries, which form a very fine network in every part of the lung. Thus the blood is pumped from the right ventricle into the pulmonary artery and distributed throughout the two lungs (Figs. 86 and 88).

We will now turn to the left side of the heart, and notice the general arrangement of its great vessels. Four veins, called the pulmonary veins, open into the left auricle, two from each lung. These veins start from very minute vessels the continuation of the capillaries of the pulmonary artery. They form larger and larger vessels until they become two large veins in each lung, and pour their contents into the left auricle. Thus the pulmonary artery carries venous blood from the right ventricle _to_ the lungs, as the pulmonary veins carry arterial blood _from_ the lungs to the left auricle.

From the left ventricle springs the largest arterial trunk in the body, over one-half of an inch in diameter, called the aorta. From the aorta other arteries branch off to carry the blood to all parts of the body, only to be again brought back by the veins to the right side, through the cavities of the ventricles. We shall learn in Chapter VIII.

that the main object of pumping the blood into the lungs is to have it purified from certain waste matters which it has taken up in its course through the body, before it is again sent on its journey from the left ventricle.

187. The Arteries. The blood-vessels are flexible tubes through which the blood is borne through the body. There are three kinds,--the arteries, the veins, and the capillaries, and these differ from one another in various ways.

The arteries are the highly elastic and extensible tubes which carry the pure, fresh blood outwards from the heart to all parts of the body.

They may all be regarded as branches of the aorta. After the aorta leaves the left ventricle it rises towards the neck, but soon turns downwards, making a curve known as the arch of the aorta.

From the arch are given off the arteries which supply the head and arms with blood. These are the two carotid arteries, which run up on each side of the neck to the head, and the two subclavian arteries, which pa.s.s beneath the collar bone to the arms. This great arterial trunk now pa.s.ses down in front of the spine to the pelvis, where it divides into two main branches, which supply the pelvis and the lower limbs.

The descending aorta, while pa.s.sing downwards, gives off arteries to the different tissues and organs. Of these branches the chief are the coeliac artery, which subdivides into three great branches,--one each to supply the stomach, the liver, and the spleen; then the renal arteries, one to each kidney; and next two others, the mesenteric arteries, to the intestines. The aorta at last divides into two main branches, the common iliac arteries, which, by their subdivisions, furnish the arterial vessels for the pelvis and the lower limbs.

[Ill.u.s.tration: Fig. 72.--Left Cavities of the Heart.

A, B, right pulmonary veins; with S, openings of the veins; E, D, C, aortic valves; R, aorta; P, pulmonary artery; O, pulmonic valves; H, mitral valve; K, columnae carnoeae; M, right ventricular cavity; N, interventricular septum.

The flow of blood in the arteries is caused by the muscular force of the heart, aided by the elastic tissues and muscular fibers of the arterial walls, and to a certain extent by the muscles themselves. Most of the great arterial trunks lie deep in the fleshy parts of the body; but their branches are so numerous and become so minute that, with a few exceptions, they penetrate all the tissues of the body,--so much so, that the point of the finest needle cannot be thrust into the flesh anywhere without wounding one or more little arteries and thus drawing blood.

188. The Veins. The veins are the blood-vessels which carry the impure blood from the various tissues of the body to the heart. They begin in the minute capillaries at the extremities of the four limbs, and everywhere throughout the body, and pa.s.sing onwards toward the heart, receive constantly fresh accessions on the way from myriad other veins bringing blood from other wayside capillaries, till the central veins gradually unite into larger and larger vessels until at length they form the two great vessels which open into the right auricle of the heart.

These two great venous trunks are the inferior vena cava, bringing the blood from the trunk and the lower limbs, and the superior vena cava, bringing the blood from the head and the upper limbs. These two large trunks meet as they enter the right auricle. The four pulmonary veins, as we have learned, carry the arterial blood from the lungs to the left auricle.

[Ill.u.s.tration: Fig. 73.

A, part of a vein laid open, with two pairs of valves; B, longitudinal section of a vein, showing the valves closed.

A large vein generally accompanies its corresponding artery, but most veins lie near the surface of the body, just beneath the skin. They may be easily seen under the skin of the hand and forearm, especially in aged persons. If the arm of a young person is allowed to hang down a few moments, and then tightly bandaged above the elbow to r.e.t.a.r.d the return of the blood, the veins become large and prominent.

The walls of the larger veins, unlike arteries, contain but little of either elastic or muscular tissue; hence they are thin, and when empty collapse. The inner surfaces of many of the veins are supplied with pouch-like folds, or pockets, which act as valves to impede the backward flow of the blood, while they do not obstruct blood flowing forward toward the heart. These valves can be shown by letting the forearm hang down, and sliding the finger upwards over the veins (Fig. 73).

The veins have no force-pump, like the arteries, to propel their contents towards their destination. The onward flow of the blood in them is due to various causes, the chief being the pressure behind of the blood pumped into the capillaries. Then as the pocket-like valves prevent the backward flow of the blood, the pressure of the various muscles of the body urges along the blood, and thus promotes the onward flow.

The forces which drive the blood through the arteries are sufficient to carry the blood on through the capillaries. It is calculated that the onward flow in the capillaries is about 1/50 to 1/33 of an inch in a second, while in the arteries the blood current flows about 16 inches in a second, and in the great veins about 4 inches every second.

[Ill.u.s.tration: Fig. 74.--The Structure of Capillaries.

Capillaries of various sizes, showing cells with nuclei]

189. The Capillaries. The capillaries are the minute, hair-like tubes, with very thin walls, which form the connection between the ending of the finest arteries and the beginning of the smallest veins. They are distributed through every tissue of the body, except the epidermis and its products, the epithelium, the cartilages, and the substance of the teeth.

In fact, the capillaries form a network of the tiniest blood-vessels, so minute as to be quite invisible, at least one-fourth smaller than the finest line visible to the naked eye.

The capillaries serve as a medium to transmit the blood from the arteries to the veins; and it is through them that the blood brings nourishment to the surrounding tissues. In brief, we may regard the whole body as consisting of countless groups of little islands surrounded by ever-flowing streams of blood. The walls of the capillaries are of the most delicate structure, consisting of a single layer of cells loosely connected. Thus there is allowed the most free interchange between the blood and the tissues, through the medium of the lymph.

The number of the capillaries is inconceivable. Those in the lungs alone, placed in a continuous line, would reach thousands of miles. The thin walls of the capillaries are admirably adapted for the important interchanges that take place between the blood and the tissues.

190. The Circulation of the Blood. It is now well to study the circulation as a whole, tracing the course of the blood from a certain point until it returns to the same point. We may conveniently begin with the portion of blood contained at any moment in the right auricle. The superior and inferior venae cavae are busily filling the auricle with dark, impure blood. When it is full, it contracts. The pa.s.sage leading to the right ventricle lies open, and through it the blood pours till the ventricle is full. Instantly this begins, in its turn, to contract. The tricuspid valve at once closes, and blocks the way backward.

The blood is now forced through the open semilunar valves into the pulmonary artery.