THE

INTERNATIONAL CYCLOPÆDIA.

IRCULATION, in anatomy and physiology, is the term used to designate the course of the blood from the heart to the most minute blood-vessels (the capillaries, q.v), and from these back to the heart.

To simplify the consideration of the subject, we shall consider-1. The anatomy of the organs of circulation-and, 2. The physiology of the circulation.

1. The organs of C. consist of the heart, arteries, veins, and capillaries. The course of the blood through these organs will be best elucidated by the aid of a diagram, which is equally applicable for all other mammals as well as for man, and for birds. The

shaded part of fig. 1 represents structures filled with impure or venous blood, while the unshaded portion represents structures in which pure, oxygenated, arterial blood occurs. In this diagram we observe a dotted circle, representing a closed bag or sac, termed the pericardium, and inclosing the four cavities c, v, c', v', of which the heart is composed. Two of these cavities, c and c', are for the purpose of receiving the blood as it flows into the heart, and are termed the auricles; while the two cavities and v' are for the purpose of propelling the blood through the lungs and general system respectively, and are termed the ventricles. The vessels that transport blood into the auricles are termed veins, and the vessels through which the blood is driven onwards from the ventricles are known as arteries (q.v.). The diagram further shows that what we commonly term the heart is in reality two distinct hearts in apposition with each other-one, shaded in the figure, which is called the right, or venous, or pulmonary heart; and the other, unshaded, which is called the left, or arterial, or systematic heartthe last name having been given to it, because the blood is sent from it to the general system; just as the right heart is termed pulmonary from its sending blood to the lungs. We will now trace the course of the blood as indicated by the arrows in this diagram, commencing with the right auricle, c. The right auricle contracting upon the venous or impure blood with which we suppose it to be filled, drives its contents onwards into the right ventricle e, through an opening between these two cavities, called the right auriculo-ventricular opening, which is guarded by a valve named the tricuspid-from its being composed of three-pointed membranous expansions-which almost entirely prevents the regurgitation or reflux of the blood from the ventricle into the auricle. The ventricle v being now filled, contracts, and as the blood cannot return into the auricle, it is driven along the shaded vessel, the dividing branches of which are indicated by f. This vessel is known as the pulmonary artery, and conveys the blood to the lungs. At its commencement, it is guarded by valves, termed, from their shape, the semilunar pulmonary valves, which entirely prevent the blood which has once been propelled into the pulmonary artery from re-entering the ventricle. The pulmonary artery gradually divides into smaller and smaller branches, which ultimately merge into capillaries. In these capillaries, which are freely distributed over the interior of all the air-cells (of which the lung is mainly composed), the venous blood is brought in contact with atmospheric air, gives off its carbonic acid gas (which is its principal impurity), and absorbs oxygen, by which processes it is converted into pure or arterial blood. The capillaries, b, in which the blood is arterialized, gradually unite to form minute veins, which, again, join to form larger vessels, until finally the blood is collected into a small number of vessels known as pulmonary veins, which pour their contents into the left auricle. Only one such vessel, g, is shown in the figure, because the main object of this diagrammatic scheme is to illustrate the mode and general direction in which the blood circulates not to indicate

h, heart, right ventricle; v', left ventricle; c, right auricle; c', left au ricle; a, aorta; d, vena cava e, greater circulation; b, smaller circulation; f, pulmonary artery; g, pulmonary veins.

the special vessels through which it flows in different parts of the body. The actual number of the pulmonary veins is four-viz., two from each lung. The blood, now fitted for the various purposes of nutrition, enters the left auricle, c', which by its contraction propels it into the left ventricle, v', through the left auriculo-ventricular opening. This opening, like the corresponding one in the right heart, is guarded by a valve which, from its form, is termed the mitral valve, and which entirely prevents the reflux of the blood. The left ventricle, v', contracts and drives its contents into the large artery, a, which represents the aorta-the great trunk which, by means of its various branches (none of which are indicated in the diagram), supplies every portion of the body, from the crown of the head to the soles of the feet, with pure arterial blood. From the aorta and its various subdividing branches, the blood passes into the capillaries, e, which occur in every part of the system; in these capillaries it undergoes important changes, which may be considered as almost exactly the reverse of those which occur in the pulmonary capillaries; it parts with its oxygen, becomes charged with carbonic acid, and, as it leaves the capillaries, and enters the minute veins formed by their union, presents all the characters of venous blood. The veins gradually unite till they form two large trunks, termed the superior and inferior vena cava, which pour their contents into the right auricle-the point from which we started. Only one of these great veins, d, is indicated in the diagram. We thus perceive that there is a complete double C.-that there is a lesser C. effected by the blood in its passage from the right to the left heart through the lungs; and that there is a greater C. effected by that fluid in its passage from the left heart through the system generally to the right heart.

From the above simple ideal scheme, we proceed to the consideration of the more complicated arrangements by which the C. is actually effected in man and the higher animals.

The heart is situated in very nearly the center of the cavity of the chest or thorax, as it is termed in anatomy, between the lungs, behind the breast-bone, or sternum, in front of the vertebral column, and above the diaphragm, on which it obliquely rests. Its

form is somewhat conical, the lower end tapering almost to a point, and directed rather forwards and to the left. This lower portion alone is movable, and, at each contraction of the heart, it is tilted forwards, and strikes against the walls of the chest between, in man, the fifth and sixth ribs, or a little below the left nipple. All the large vessels connected with the heart-the vena cava, the pulmonary artery, and the aorta-arise from its base (see fig. 2), and serve, from their attachment to the neighboring parts, to keep that portion of k it fixed. Indeed, these vessels may be regarded as suspending the heart in the cavity, which is lined by a smooth serous membrane, which, near the top, is reflected downwards over the roots of the great vessels, and covers the whole of the outer surface of the heart. These two smooth serous surfaces-one lining the cavity, the other investing the heart-are kept moist by a fluid which they secrete, and by this arrangement, friction may be regarded as reduced to its minimum. The cavity or sac in which the heart lies is called the pericardium. Like all serous membranes, it is a closed sac, and, as it may not be easy for the non-professional reader to understand the relative position of the heart, which is at the same time surrounded by and external to this membrane, we may observe that the head in an old-fashioned double night-capwhich is a closed bag-is in much the same position as the heart in the pericardium; it is inside the night-cap, but not in the cavity which intervenes between its two layers.

FIG. 2.-THE LUNGS, HEART, AND PRINCIPAL
BLOOD-VESSELS IN MAN.

a, h, veins from the right and left arms; b,
f, right and left jugular veins, returning
the blood from the head and neck-these
four veins unite to form a single trunk,
the vena cava superior, which enters the
right auricle, 1; c, e, the right and left
carotid arteries, the latter rising directly
from the arch of the aorta, a', the former
from a short trunk called the arteria in
nominata; g, the left subclavian artery,
rising directly from the aorta, while the
right subclavian springs from the arteria
innominata; d, the trachea or windpipe;
i, k, the right and left lungs; 1, 1', the
right and left auricles; p, the right ven-
tricle: o, the apex of ventricle; m, the
inferior or ascending vena cava; n, the
descending aorta, emerging from behind

The substance of the heart is essentially muscular. The fibers run in different directions, longitudinally and transversely, but most of the heart; q, the pulmonary artery. them obliquely; many pass over the apex from one side of the heart to the other; and all are so involved as to render it very difficult to unravel them. In consequence of this arrangement, the fibers, by their contraction, seem simultaneously to diminish each cavity in all directions, and thus serve most effi ciently to drive the blood onwards. The size of the heart has been estimated as about that of the closed fist of the same individual. Its weight, as compared with that of the body, was determined by Dr. Clendinning to be 1:160 in the male, and 1:150 in the female. The same physician carefully examined nearly four hundred hearts of persons

of both sexes, and determined the average weight at about 9 oz. avoirdupois, while Dr. John Reid found the average weight of the male heart to be a little more than 11 ozs., and that of the female heart to be a little above 9 ozs.

In our ideal sketch of the organs of C. (fig. 1), we have indicated the different cavities into which the heart is divided. In fig. 3 there is represented a section of the human heart, which is sufficiently like the reality to give the reader a fair idea of the position of its various parts. The two theoretical hearts, which were nearly in contact in fig. 1, are here fused into a single organ, but the division of the two sides is still as complete, in so far as the functions of the heart are concerned, as in the ideal scheme. We see a strong vertical partition separating the entire heart into two halves, which are very similar to each other. In the accompanying figure (fig. 4), we have a representation of all these valves-the auricles having been removed so as to give a distinct view of the upper surface of the ventricles. The tricuspid and mitral valves, which are entirely closed-the two ventricles contracting simultaneously-are represented by 1 and 3 respectively; while the pulmonary and aortic semilunar volves, which, when closed, always present a concave surface towards the lungs, are indicated by 4 and 5. The walls of the ventricles are much thicker than those of the auricles, and those of the left ventricle are about four times as thick as those of the right; the amount of muscular tissue being, in all these cases, proportional to the work to be done. All details regarding the anatomy of the heart, except such as bear directly upon the C., would be out of place in this article, and we shall, therefore, omit all notice of many structures which present themselves on opening its various cavities. We will merely add, that

n

HEART:

f, b, the two vena cava, opening into d, the right auricle; c, the tricuspid valve; a, the right ventricle, from which proceeds the pulmonary artery, dividing into branches g and i, going to the right and left lung respectively; e, e', the pulmonary veins (two from either lung), entering into the left auricle, k; 1, the mitral valve; m, the left ventricle, from which proceeds the aorta, whose arch is indicated by h, and the descending portion by n, none of its branches being indicated in this figure; o, the partition, or septum, between the right and left hearts.

FIG. 4.-VALVES OF THE HEART AND ARTERIES.

Upper surface of the heart, the auricles having been removed. In this figure the heart is turned in such a position that the anterior surface lies lowermost; hence the apparent discrepancy of the right auriculo-ventricular orifice lying on the left side of the diagram.

1, Right auriculo-ventricular orifice, obliterated by the tricuspid valve; 2, fibrous ring surrounding this orifice; 3, left auriculo-ventricular orifice, surrounded by a ring, and closed by the mitral valve; 4, orifice leading into the aorta from the left ventricle, closed by the semilunar valves; 5, orifice leading into the pulmonary artery from the right ventricle, also provided with three semilunar valves.

the heart receives the arterial blood necessary for its own nutrition from the coronary arteries, two trunks which are given off by the aorta immediately above the semilunar valves; and that this blood having discharged its function, is carried back to the right auricle by the coronary veins; this blood obviously having the shortest possible systemic circulation.

Since all the arterial blood leaves the heart through the aortic opening, in tracing its course to the different parts of the system, we obviously have only to follow the aorta to its final branches. Referring to the article AORTA, where the principal branches of that great organ are indicated, it is sufficient, without further anatomical details, to say that the final ramifications of the arteries distribute the arterial blood to the capillaries (q.v.), which pervade every part of the body.

The veins, like the arteries, are found in nearly every tissue; they commence by minute plexuses (an anatomical term for a network-like arrangement), which communicate with the capillaries. Branches from these plexuses uniting together, form small venous trunks, which, by joining, increase in size as they pass onward towards the heart. If we except certain venous structures (called sinuses) occurring in the interior of the skull, we may divide the veins into two sets-the superficial or cutaneous, and the deep veins.

The deep veins accompany the arteries, and are usually inclosed in the same sheath of cellular tissue with them. In the case of the smaller arteries, they generally exist in pairs, one on each side the artery, and are called vena comites, while the larger arteries have usually only one accompanying vein.

8

The superficial veins occur immediately beneath the integument; they not only return the blood from the skin and adjacent structures, but communicate with the deep veins.

All the veins finally unite into two large trunks, termed the superior and inferior vena cava, which open into the right auricle of the heart; the superior vena cava being formed by the union of the veins which return the blood from the head and neck (the jugulars) with those which convey it from the arms (the subclavians), as shown in fig. 2; while the inferior vena cava (also shown in the same figure) receives the blood from the lower extremities, the trunk, and the abdominal and pelvic viscera. We must refer to the article VEIN for the structure of the walls of this part of the circulating system. There is only one point that imperatively requires notice here-viz., that while the arterial system presents no valves, except at the points where the two great trunks leave the heart, the veins contain a great number of valves, which are formed by a doubling of their lining membrane, and resemble pocket-like folds or pouches, which allow the blood free passage towards the heart, but prevent its reflux.

There is one part of the venous C. which, from its great importance, requires special notice-viz., that of the venous blood of the spleen, prancreas, stomach, and intestinal canal. The blood supplied to these organs by the coeliac axis and the two mesenteric arteries is not returned directly to the vena cava, and thence to the heart, as occurs in other parts of the system. The veins of these organs unite together into one large vessel, called the vena porta, which, entering the liver, branches out again like an artery, and finally subdivides into a capillary network that permeates the whole of its mass. It is from the venous blood, as it traverses these capillaries, that the bile is secreted. This portal blood, together with the blood of the hepatic artery, after it has become venous, is finally carried off by the hepatic veins (usually three in number), which open into the inferior vena cava. * Thus the blood which flows through the portal vein passes through two sets of capillaries, between the period of its leaving the aorta and entering the vena cava.

Our knowledge of the true course of the C.-viz., that the blood propelled from the left side of the heart, after traversing the arteries, returned by the veins to the right side of the heart; and the blood of the right side, passing through the pulmonary artery, traversed the lungs, and returned by the pulmonary veins to the left auricle-is of comparatively recent date. Harvey's celebrated work, Exercitatio de Motu Cordis et Sanguinis, was not published till 1628, although there is good reason to believe that it was written nine or ten years previously. Before the appearance of this celebrated work, which marks an epoch in physiological science, the views that were held on this subject were so vague that it is unnecessary for us to enter into any notice of them. (The reader who takes an interest in this point is referred to Dr. Willis's Life of Harvey, prefixed to his translation of Harvey's works, for the Sydenham society.) In one point, Harvey's proof of the course taken by the blood was defective; the microscope had not then revealed the existence of the capiflaries, and he was consequently altogether at fault as to the mode by which the blood passed from the arteries to the veins. By a strange coincidence, Malpighi, who discovered the corpuscles by which the motion of the blood in the capillaries can be traced, was born in the course of the very year (1628) in which Harvey's work was published.

The double C. which we have described, is the course performed by the blood from the time of birth during the whole period of life. The C. of the blood, however, begins before birth-indeed, at a very early period of intra-uterine or fetal existence; and the circumstance that before birth the lungs do not act as organs of respiration, induces a very important modification in the course of the blood in fetal life, which will be described under FŒTUS.

A

B 23

b

C

D

We now leave for the present the C. in man, and proceed to notice some of the leading peculiarities of the C. in other animals. In the warm-blooded animalsmammals and birds-the course of the blood is essentially the same as in man, for in all these animals the heart, like the

FIG. 5.-DIAGRAM OF CERTAIN VARIETIES IN THE ORI- adult human heart, possesses four distinct

GIN OF THE MAIN TRUNKS FROM THE ARCH OF THE
AORTA:

A, Man; B, the Ruminants; C, Dolphin and Bats;
D, the Elephant. 1, the right subclavian; 2, right
carotid; 3, left carotid; 4. left subclavian; a,
ascending aorta; b, descending aorta.

cavities. In form, however, it presents certain peculiarities in some of the mammalia. and less elongated than in man. It is generally more rounded In the cetacea, it is very broad and flat; and in at least one genus, the dugong, the right and left ventricles are separated by a deep fissure. In some herbivorous mammals, as in the ox, sheep, goat, etc., a cruciform ossification, called the bone of the heart, is found in the septum between the ventri

*In fishes, not only the blood of the intestines, but that of the posterior part of the body, enters this portal system, which is distributed in this class of animals both to the kidneys and to the liver.