134

COURSE OF THE CIRCULATING BLOOD.

If that be accomplished, the circulation is restored, and the heart proceeds with its duty. And for these reasons, I believe that in many cases success would be had, where failures are now experienced, if, instead of resorting to atmospheric air, pure oxygen gas or protoxide of nitrogen were administered.

In the more highly-developed organisms the objects of the circulation are threefold: 1st. To minister to the nutrition of the system; 2d. To introduce oxygen; 3d. To remove the products of waste. In man, these various results are accomplished by several different arrangements: 1st. The greater, or systemic circulation; 2d. The less, or pulmoclasses of nary circulation; 3d. The portal circulation; 4th. The Malpicirculation. ghian circulation, &c.

Different

Course of the

temic and pul

The course taken by the blood is as follows. Leaving the left ventricle of the heart, it passes into the aorta, and is distributed blood in its sys- by the ramifications thereof, constituting the systemic artemonary circu- ries, to all parts of the system. It moves onward through lations. the capillaries, which may at once be considered as the terminal ramifications of the arteries and the commencing tubelets of the veins. These, converging into larger and larger venous trunks, the systemic veins, deliver it into the ascending and descending venæ cavæ, from which it flows into the right auricle, and from thence into the right ventricle of the heart. From thence it is driven into the pulmonary artery, to be distributed to the lungs, and, coming therefrom along the pulmonary veins, reaches the left auricle, and from thence it gains the left ventricle, which was its starting-point.

Distribution of

blue blood.

nary.

In the pulmonary veins, the left cavities of the heart, and in the systemic arteries, the blood is crimson. In the systemic veins, crimson and of the right cavities of the heart, and pulmonary artery and its branches, it is blue. The change from crimson to blue takes place in the systemic capillaries, and from blue to crimson in the pulmoThe systemic, or greater circulation, is considered as beginning at the left ventricle and ending at the right auricle; the pulmonary, or less circulation, begins at the right ventricle and ends at the left auricle. This double course is sometimes, among authors, illustrated by likening it to the figure 8, the upper loop representing the pulmonary, the lower the systemic circulation, and the heart placed at the nodal point. As has just been remarked, there are other subordinate circulations, but of these only one need attract our attention at present-it circulation. is the portal. This originates in a system of capillaries, the veins belonging to the digestive apparatus, which, converging rapidly together, form a common trunk, the portal vein. This at once ramifies like an artery in the substance of the liver. From the resulting capillaries, the portal blood passes into the commencing capillaries of the hepat

The portal

ORIGIN OF THE HEART.

135

by that of a

ic veins, which empty into the inferior vena cava, and so it reaches the general circulation. The physical peculiarity of the portal circulation is, that it commences in a capillary system, and ends in one, without the intervention of any central organ of impulse, or heart. At a very early period, comparative anatomists were struck with Portal circulathe analogy between the portal circulation in man and the tion illustrated systemic circulation of fishes, both being carried on in the fish. same way, that is, without a heart. In fishes, the heart is a branchial, respiratory, or pulmonary one. Their systemic circulation, or circulation of crimson blood, commences in the capillaries of the respiratory apparatus, the gills; a convergence takes place into an aorta, which ramifies into systemic capillaries. So the great circulation in these tubes is accomplished without any heart. It is scarcely necessary to point out the bearing of such a fact on the theories of the movement of the blood.

Fig. 54.

Diagram of fish circulation.

In Fig. 54 is a diagram of the circulation of a fish; a, is the auricle; b, the ventricle; c, the branchial or pulmonary artery; e, e, the branchial or pulmonary veins, bringing blood from d, the branchia, and converging directly to f, the aorta, which distributes the systemic blood. This is collected into a vena cava, g, and so brought to the auricle, a. There is therefore no systemic heart.

The further discussion of this subject will be continued as follows: We shall describe, 1st, the construction and action of the heart; 2d, of the arteries; 3d, of the capillaries; 4th, of the veins. We shall then present a view of the combined result of these various mechanisms.

The heart.

1st, The Heart. The first appearance of the heart is as a cavity arising in a collection of cells, by deliquescence or separation of the central ones. At this early period, and even before the cavity has fairly formed, pulsation may be observed. The organ soon assumes a tubular form; and this,

Fig. 55.

Rudimentary heart.

becoming curved, as shown in Fig. 55, differentiates into three compartments, with arterial and venous connections; 1, the venous trunks; 2, the auricle; 3, the ventricle; 4, the bulbus arteriosus. The form to be

eventually assumed is foreshadowed in the manner in which the curved tube develops, the arch of the curve, 2, bulging so as to form a conical ventricle. This tri-chambered heart remains permanent in fishes, as seen in the preceding figure (54), of which c is the third chamber. But in birds and mammals, the aortic bulb merges into the ventricle, through which, as well as through the auricle, a septum or partition is established, and

136

Fig. 56.

Diagram of single heart.

STRUCTURE OF THE HEART.

thus a double heart, or one of four chambers, arises.

The diagram, Fig. 56, represents a double-chambered heart, d being its auricle, e the ventricle, c, c, the veins converging to the auricle, a the aorta or main artery passing from the ventricle. The course of the blood is indicated by the arrows.

The heart with four cavities may be considered as arising from the conjunction of a pair of the preceding form, with their efferent and afferent tubes, or arteries and veins, so modified or arranged that the right heart receives its blood from the

system in an auricle, from which it passes into a ventricle, and thence to

Fig. 57.

the lungs. From the lungs, after aeration, this blood is brought to the auricle of the left heart, thence into its ventricle, and thence to the aorta. Though all four cham'bers are generally coalesced into one conical form, the heart of the dugong, Fig. 57, presents the true typical structure; E is the right or pulmonary ventricle, L the left or systemic ventricle, their apices being quite apart; D is the right or systemic auricle, F the pulmonary artery, K the left or pulmonary auricle, and A the aorta.

Fig. 58 is the anatomy of the human heart as viewed upon the right

side, the figure and description being from Dr. E. Wilson. 1, the cavity of the right auricle; 2, the appendix auricula; 3, the superior vena cava, opening into the upper part of the right auricle; 4, inferior vena cava; 5, the fossa ovalis; the prominent ridge surrounding it is the annulus ovalis; 6, the Eustachian valve; 7, the opening of the coronary vein; 8, the coronary valve; 9, the entrance of the auriculo-ventricular opening; a, the right ventricle; b, c, the cavity of the right ventricle, on the walls of which the columnæ carnea are seen; c is placed in the channel leading upward

STRUCTURE OF THE HEART.

137

to the pulmonary artery, d; e, f, the tricuspid valve: e is placed on the anterior curtain, and f on the right curtain; g, the long columna carnea, to the apex of which the anterior and right curtains of the tricuspid valve are connected by the chorda tendineæ; h, the long moderator band; i, the two columnæ carnea of the right curtain; k, the attachment by chordæ tendineæ of the left limb of the anterior curtain; 7, 7, chorda tendineæ of the fixed curtain of the valve; m, the valve of the pulmonary artery: the letter of reference is placed on the inferior semilunar segment; n, the apex of the right appendix auricula; o, the left ventricle; p, the ascending aorta; q, its arch, with the three arterial trunks which arise from the arch; r, the descending aorta. Fig. 59 exhibits the view of the organ on its left side. Like the preceding, the figure and description are from Dr. Wilson.

Fig. 59.

Human heart on the left side.

1, cavity

of the left auricle: the number is placed on that portion of the septum auricularum corresponding with the centre of the fossa ovalis; 2, cavity of the appendix auriculæ; 3, opening of the two right pulmonary veins; 4, the sinus into which the left pulmonary veins open; 5, the left pulmonary veins; 6, the auriculoventricular opening; 7, the coro

nary vein, lying in the auriculo-ventricular groove; 8, the left ventricle; 9, 9, the cavity of the left ventricle. The numbers rest on the septum ventriculorum. a, the mitral valve: its flaps are connected by chorda tendineæ to b, b, b, the columnæ carneæ; c, c, fixed columnæ carneæ, forming part of the internal surface of the ventricle; d, the arch of the aorta, from the summit of which the three arterial trunks of the head and upper extremities are seen arising; e, the pulmonary artery; f, the obliterated ductus arteriosus; g, the left pulmonary artery; h, the right ventricle; i, the point of the appendix of the right auricle.

Externally, the heart is covered by a serous membrane, pericardium,

Fig. 60.

Muscular fibres of the heart.

and in its interior is sheathed by the endocardium, an extension of the interior coat of the great blood-vessels. Though its movements are wholly involuntary, its muscular fibres are of the transversely striated kind. They are about one third less in diameter than those of voluntary muscles generally,

138

COURSE OF THE BLOOD IN THE HEART.

and are especially characterized by their disposition to anastomose with one another, as represented in Fig. 60. In the ventricles, the arrangement is such that the fibres of the external and internal surfaces decussate.

The motions of the heart consist in the relaxations and contractions of Relaxations the muscular walls of its cavities. The two auricles contract and contrac- at the same moment, as do also the two ventricles, but the contractions of the auricles coincide with the relaxations of the

tions of the

heart.

ventricles.

blood in the

heart, and

the valves.

The course of the blood through the heart is this. The venous blood, Course of the brought by the ascending and descending cava, flows into the right auricle as it is dilating, and for the moment pushes movements of forward to the ventricle, but the auricle, being of less capacity than the ventricle, is filled to distention first; at this instant it contracts, forcing its contents past the tricuspid valve into the ventricle, and fills it completely. The blood can not regurgitate into the veins to any extent while this is going on, because of the almost perfect closure of their valves. The right ventricle now commences to contract; its fleshy columns shorten so as to pull upon the tendinous cords attached to the flaps of the tricuspid valve: this enables the blood to get behind them, and they quietly close the aperture between the auricle and ventricle; the closure is not, however, under all circumstances, perfect, the mechanism being such as to permit leakage or regurgitation to a limited extent. The blood now rushes into the pulmonary artery, passing by its semilunar valves, which, the moment the ventricular pressure ceases, shut, so as to prevent any return to the heart.

Having passed through the lungs and been submitted to the air, the blood now returns to the left auricle, which forces it into the left ventricle, the action on this side of the heart being the same as on the other; the mitral valve, which closes the opening from the auricle into the ventricle, is worked in the same manner as the tricuspid, and the blood is pressed into the aorta, the semilunar valves of which, at that instant, shut abruptly with an audible sound, and prevent any regurgitation. In this manner the distribution to the system is accomplished.

On both sides of the heart, as soon as the auricles have finished their contraction, they begin to dilate, and continue to do so during the period that the ventricles are contracting. Thus there is an accumulation in them when the ventricles are ready to dilate, and, as soon as that occurs, the blood flows freely forward into those cavities, the complete distention of which is then accomplished by the contraction of the auricles, as before explained.

Movements of

The mode of action of the two sets of cavities is different.

the auricles The auricles contract suddenly, first at the place of junction

and ventricles.