4

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 cave, 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 cara, 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.

à, 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 ventricle; o, the apex of ventricle; m, the The substance of the heart is essentially musinferior or ascending vena cava; n, the cular. The fibers run in different directions, descending aorta, emerging from behind 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 efficiently 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

e

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.

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 tena 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 cœliac 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 capillaries, 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

b

very important modification in the course of the blood in fetal life, which will be described under FETUS.

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. In the It is generally more rounded 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.

7

cles. In the ornithorhynchus, or duck-billed platypus, the heart, in some respects, resembles that of birds. We likewise find certain varieties in the distribution of the blood-vessels. Thus, while in man the subclavian and carotid arteries arise on the right side from a short common trunk given off by the aorta, and on the left side arise directly from the aorta, we find several varieties of this arrangement in the mammalia. In the horse and the ruminants, the aorta divides at once at its origin into an anterior trunk, which gives off the carotid and subclavian arteries of both sides, and a posterior trunk for the thoracic and abdominal aorta. In the dolphin, and in some-if not allof the bats, two short trunks (arteriæ innominata) arise, and give off each a carotid and subclavian on either side. In the elephant, both carotids are given off from a single common trunk, situated midway between the two subclavians. All these, and other varieties which might be noticed, are occasionally found in man; and it may be laid down as a general rule, that when any abnormal arterial distribution is detected in the human subject, it represents the normal type in some lower mammal.

A very remarkable peculiarity in the distribution of the vascular system (both arteries and veins) is exhibited by the cetacea and other diving animals, in which the respiration, and consequently the-arterialization of the blood, is temporarily stopped. Various arteries of the trunk here assume a ramified and convoluted form, so as to constitute reservoirs capable of holding a large quantity of pure blood; while the venous trunks exhibit similar dilatations, capable of receiving and retaining for a considerable time the impure blood which has circulated through the system, and of thus preventing the right heart from being overcharged with venous blood during the temporary suspension of respiration. By means of these arterial reservoirs, the cetacea can support life under water for a quarter of an hour, or even longer.

Another peculiarity deserving of notice is, that occasionally a large artery will divide into a great number of smaller vessels, which again reunite to form a single trunk. An arrangement of this kind is known as a rete mirabile, and a good example of it occurs within the skull in long-necked grazing animals, the object being to clieck too strong a current of blood to the brain.

In birds, the heart is usually of a very large size, as compared with the bulk of the body. The trunk of the aorta is extremely short, and divides into three main branches, the central one forming the descending aorta, while the two lateral ones give off the carotid and subclavian arteries on either side. The branches of the latter give an abundant supply of blood to the powerful thoracic muscles by which the wings are moved.

In the class of reptiles, there is not a complete double C., a mixture of arterial and venous blood being sent both to the lungs and to the general system. In fig. 6, the general nature of the C. in this class is typically represented. The heart consists of two auricles and one ventricle. The impure blood which has circulated through the system is conveyed by the vena cava into the right auricle, from whence it passes into the commou ventricle. At the same time, blood which has been aerated in the lungs is poured into it from the left auricle; hence the ventricle contains an admixture of venous and arterial blood. As both a pulmonary artery and an aorta are given off by the ventricle, the latter by its contractions simultaneously drives one portion of its contents to the lungs, and another to the general system. In this way, a semi-oxygenated blood is transmitted to the various parts of the body, the only pure blood being that which is contained in the left auricle and in the veins opening into it.

h

FIG. 6.-CIRCULATION IN
REPTILES:

h, heart, inclosed in peri-
cardium; f. f, right and
left auricles; v, single
ventricle; a, aorta; b,
vena cava; C smaller
circulation; d, greater

circulation.

Although the above may be regarded as the general type of the circulating apparatus in reptiles, yet there are many modifications of it (into which we have not space to enter), which connect it on the one hand (in the case of the perennibranchiate amphibia, such as the axolotl, proteus, etc.) with that of fishes, and on the other hand (when there is a more or less perfect separation of the ventricular cavity, as in the crocodiles) with that of birds and mammals.

In the class of fishes, the circulating apparatus is far simpler than in reptiles. The heart possesses only two cavities, an auricle and a ventricle, and is traversed solely by venous blood; hence it is analogous to the right side of the mammalian heart. Venous blood is brought by veins, which correspond with our vena cava, from all parts of the system, and enters the auricle (see fig. 7); from the auricle, the blood passes into the ventricle, which is of great muscular strength; and the ventricle propels its contents through a vessel which corresponds with our pulmonary artery, and which dividing on either side into four or five branches, goes to the gills, in the capillaries of which it becomes oxygenated, by means of the air that is diffused through the water. From the filaments and fringe-like structures of the gills, it is at length collected into a large trunk, commonly called the dorsal vessel, but analogous to the aorta of mammals and birds, in so much as it supplies the whole body with arterialized blood. After passing through the systematic capillaries, the blood returns in a venous condition to the heart,

and the above process is repeated. Although the heart is simpler than in reptiles, the C. is in one sense of a higher character, in so far as pure arterial (not mixed) blood is here conveyed to all parts of the system; hence, probably, the far greater muscular energy of fishes may be explained.

in various directions.

We can only allude very briefly to the
C. in the invertebrate animals.

*

In the mollusca, we find hearts of
varying complexity, usually with one
or two auricles, and one ventricle; but
in all cases, the auricle or auricles receive
aërated blood from the respiratory organs
and pass
it to the strongly muscular
ventricle, which propels it over the body.
The heart is therefore a systemic heart.
There seem to be no capillaries in these
animals, excepting in the respiratory
organs; the blood leaving the open ends
of the arteries, passes into the inter-
stices (lacuna) of the parenchyma of the
body, from whence it is taken up by the
open mouths of the venous radicles;
hence this kind of C. is called lacunary.

In the crustacea, the form of the
heart and the number of its orifices pre-
sents several modifications; the following
is, however, the typical mode of C. of
these animals. The heart, which is here
a single cavity, is sometimes round, and
sometimes long and tubular, and is the
point of departure of the arterial sys-
tem, which consists of trunks emerging

FIG. 8.-CIRCULATION IN
THE CRUSTACEANS:

h, the heart and pericar

dium; a, the arteries; c', the systemic capillaries; c,the branchial or respiratory capillaries; b, the branchiocardiac vessels.

The blood returning from the arteries does not enter into distinct veins, but into irregular excavations in the tissues, which are termed venous sinuses; from these venous sinuses it passes to the gills, from whence it is returned to the heart in an aerated state by the branchio-cardiac canals; so that here, as in the mollusca, the heart is systemic.

It is unnecessary for us to notice the comparatively imperfect C. in insects and animals lower in the scale than those we have already considered.

We now approach the last part of the subject-the physiology of the circulation. We shall consider-1. The flow of blood through the heart; 2. The phenomena of the arterial C.; 3. The phenomena of the capillary C.; and, 4. The phenomena of the venous circulation.

1. Direct observation and experiment clearly show, that the muscular contraction of the heart is the principal source of the power by which the blood is propelled in its course. This action of the heart may be observed by opening the chest of a living animal, or, better still, of an animal deprived of sensation and motion by poison, and in which artificial respiration is kept up. It is then seen to consist of two motions-first, a contraction or systole of the auricles, and second, a corresponding contraction of the ventricles. The contraction of the auricle immediately precedes that of the ventricle, and the systole of each cavity is directly followed by its diastole or relaxation; there is then a brief period of repose, the heart exhibiting little or no motion. At the moment of the systole of the ventricles, the apex of the heart is tilted forwards, causing a pulsation against the ribs that can be felt externally.

The force exerted by the left ventricle has been so very variously estimated, that we must regard this point as still unsettled. The number of contractions of the heart of an adult in a minute is about 70 or 75; it is, however, liable to great variations, which will be noticed in the article PULSE. The sounds accompanying the heart's action, which may be readily heard by applying the ear either directly or through the medium of the stethoscope to the cardiac region, are discussed in the article HEART, SOUNDS OF THE. 2. The arteries exercise a vast influence on the movement of the blood through them, in virtue of two properties which they possess-viz., elasticity and contractility. These two endowments are not equally and uniformly possessed by the whole arterial system -elasticity (the property by which the interrupted or discontinuous force of the heart is made equable and continuous) existing chiefly in the larger trunks; while contractility-which is more required for regulating the flow of biood to particular parts-is most

*In some of the ascidians and in salpa, the following remarkable phenomenon occurs: The heart, which is extremely simple, and of course without valves, at definite intervals (of about twenty minutes) reverses the direction of its current. Before the heart changes the direction of its contractions, it remains still for a short time, and the blood-currents in the body are thus slackened in their course before they receive an impulse in the opposite direction. The vessels entering and leaving the heart thus act alternately as an aorta and as a vena cava.