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thus, however, they are of moment. When we think of the vast number of beats executed by the heart every day, the great amount of energy rendered manifest in maintaining the circulation becomes apparent, and our interest is heightened in the fact that all of this large sum of energy is liberated in the muscular tissue of the heart itself. Thus, too, the physiological significance of the diastole is accentuated as a time of rest for the cardiac muscle, as well as a necessary pause for the admission of blood into the ventricle. To disregard minor considerations, the work done at a systole will evidently depend upon the amount of the pulse-volume, of the arterial pressure overcome, and of the velocity imparted to the ejected blood. All these are variable. The work of the ventricles therefore is eminently variable.

The Heart's Contraction as a Source of Heat.-In dealing with the movement of the blood in the vessels we have seen that the energy of visible motion liberated by the cardiac contractions is progressively changed into heat by the friction encountered by the blood; and that this change is nearly complete by the time the blood has returned to the heart, the kinetic energy of each systole sufficing to drive the blood from the heart back to the heart again, but probably not being much more than is required for this purpose. Practically, therefore, all the energy of the heart's contraction becomes heat within the body itself, and leaves the body under this form. As the heart liberates during every day an amount of energy which is always large but very variable, its contractions evidently make no mean contribution to the heat produced in the body and parted with at its surface.

H. THE MECHANISM OF THE VALVES OF THE HEART.

Use and Importance of the Valves.-The discussion just concluded shows the work of the heart to be the forcible pumping of a variable pulsevolume out of veins where the pressure is low into arteries where the pressure is high. It is owing to the valves that this is possible, and so dependent is the normal movement of the blood upon the valves at the four ventricular apertures that the crippling of a single valve by disease may suffice to destroy life after a longer or shorter period of impaired circulation.

The Auriculo-ventricular Valves.-The working of the auriculo-ventricular valves (see Fig. 18) is not hard to grasp. When the pressure within the ventricle in its diastole is low, the curtains hang free in the ventricle, although probably never in close contact with its wall. As the blood pours into the ventricle, the pressure within it rises, currents flow into the space between the wall and the valve, and probably bring near together the edges of the curtains and also their surfaces for some distance from the edges. Thus, upon the cessation of the auricular systole, the supervening of a superior pressure within the ventricle probably applies the already approximated edges and surfaces of the curtains to one another so promptly that the commencing contraction of the ventricle is not attended by regurgitation into the auricle. The principle of closure is the same for the tricuspid valve as for the mitral. As the forces are exactly equal and opposite which press together the

opposed parts of the surfaces of the curtains, those parts undergo no strain, and hence are enabled to be exquisitely delicate and flexible and therefore easily fitted to one another. On the other hand, the parts of the valve which intervene between the surfaces of contact and the auriculo-ventricular ring are tough and much thicker, as they have to bear the brunt of the pressure within the contracting ventricle. As the systole of the ventricle increases, the auriculo-ventricular ring probably becomes smaller, and the curtains of the valve probably become somewhat fluted from base to apex, so that their line of contact is a zig-zag. At the same time their surfaces of contact may increase in

extent.

Tendinous Cords and their Uses.-The structure so far described is wonderfully effective because it is combined with an arrangement to prevent a reversal of the valve into the auricle, which otherwise would occur at once. This arrangement consists in the disposition of the tendinous cords, which act

[graphic]

FIG. 18.-The left ventricle and aorta laid open, to show the mitral and aortic semilunar valves (Henle).

as guy-ropes stretched between the muscular wall of the ventricle and the valve, whether mitral or tricuspid. These cords are tough and inelastic, and, like the valve, are coated with the slippery lining of the heart. They are stout where they spring from the musele, but divide and subdivide into branches, strong but sometimes very fine, which proceed fan-wise from their

stem to their insertions (see Fig. 18). These insertions are both into the free margin of the valve and into the whole extent of that surface of it which looks toward the wall of the ventricle, quite up to the ring. By means of this arrangement of the cords each curtain is held taut from base to apex throughout the systole of the ventricles, the opposed surfaces being kept in apposition, and the parts of the curtains between these surfaces and the ring being kept from bellying unduly toward the auricle. Each curtain is held sufficiently taut from side to side as well, because the tendinous cords inserted into one lateral half of the curtain spring from a widely different part of the wall of the heart from those of the other lateral half of it (see Fig. 18). At all times, therefore, even when the walls of the ventricle are most closely approximated during systole, the cords may pull in slightly divergent directions upon the two lateral halves of each curtain. This arrangement of the cords may also cause them, when taut, to pull in slightly convergent directions upon the contiguous lateral halves of two neighboring curtains and thus to favor the pressing of them together (see Fig. 18).

Papillary Muscles and their Uses.-In the left ventricle the tendinous cords arise in two groups, like bouquets, from two teat-like muscular projections which spring from opposite points of the wall of the heart, and which are called the "papillary muscles" (see Fig. 18). One of these gives origin to the cords for the right half of the anterior and for the right half of the posterior curtain; the other papillary muscle gives rise to the cords for the left halves of the two curtains. Each papillary muscle is commonly more or less subdivided (see Fig. 18). The same principles are carried out, but less regularly, for the origins of the tendinous cords of the more complex tricuspid valve. Various opinions have been held as to the use of the papillary muscles. It seems probable that during the change of size and form wrought in the ventricle by its systole, the origins of the tendinous cords and the auriculoventricular ring tend to be approximated and the cords to be slackened in consequence. Perhaps this is checked by a compensatory shortening of the papillary muscles, due to their sharing in the systolic contraction of the muscular mass of which they form a part. Observations have been made which have been interpreted to mean that the papillary muscles begin their contraction slightly later and end it slightly earlier than the mass of the ventricle.'

Semilunar Valves.-The anatomy and the working of the semilunar valves are the same in the aorta as in the pulmonary artery, and one account will answer for both valves. Each valve is composed of three entirely separate segments, set end to end within and around the artery just at its origin from the ventricle. The attachments of the segments occupy the entire circumference of the vessel (Fig. 18). Like the tricuspid and mitral valves, each semilunar segment is composed of a sheet of tissue which is tough, thin, supple, and slippery; but the semilunar valves differ from the tricuspid and 1C. S. Roy and J. G. Adami: "Heart-beat and Pulse-wave," The Practitioner, 1890, i.

p. 88.

mitral, not only in the complete distinctness of their segments, but also in their mechanism. The tendinous cords are wholly lacking, and each segment depends upon its direct connection with the arterial wall to prevent reversal into the ventricle during the diastole of the latter. If the artery be carefully laid open by cutting exactly between two of the segments, each of the three is seen to have the form of a pocket with its opening turned away from the heart (see Fig. 18). Behind each segment, the artery is dilated into one of the hollows or "sinuses" of Valsalva.' As the valve lies immediately above the base of the ventricle the segments rest upon the top of the thick muscular wall of the latter, which affords them a powerful support (see Fig. 19). Each segment is attached by the whole length of its longer edge to the artery, while the free margin is formed by the shorter edge. It is this arrangement which renders reversal of a segment impossible (see Fig. 18).

FIG. 19.-Diagram to illustrate the mechanism of the semilunar valve.

FIG. 20.-Diagram to illustrate the mechanism of the semilunar valve and corpora Arantii.

While the blood is streaming from the ventricle into the artery, the three segments are pressed away by the stream from the centre of the vessel, but never nearly so far as to touch its wall. At all times, therefore, a pouch exists behind each segment, which pouch freely communicates with the general cavity of the artery. As the ventricular systole nears its end, the ventricular cavity doubtless becomes narrowed just below the root of the artery, and with it the arterial aperture itself, while currents enter the sinuses of Valsalva. Thus for a double reason the three segments of the valve are approximated, and probably the last blood pressed out of the ventricle issues through a narrow chink between them. The instant that the pressure in the ventricle falls below the arterial pressure, the three segments must be brought together by the superior pressure within the artery, and tightly closed by its forcible recoil, without regurgitation having occurred in the process (see Figs. 19, 20).2

Lunulæ and their Uses.-Each segment of a semilunar valve, when closed, is in firm contact with its fellows not only at its free margin but also over a considerable surface, marked in the anatomy of the segment by the two "lunula" or little crescents, each of which occupies the surface of the segment from one of its ends to the middle of its free margin, the shorter edge

1 Named from the Italian physician and anatomist Valsalva of Bologna, born in 1666. * L. Krehl: "Beiträge zur Kenntniss der Füllung und Entleerung des Herzens,” Abhandlungen der math.-physischen Classe der k. sächsischen Gesellschaft der Wissenschaften, 1891, Bd. xvii. No. 5, S. 360.

of the lunula being one-half of the free margin of the segment (see Fig. 18). Over the surface of each lunula each segment is in contact with a different one of its two fellows (see Fig. 20). The firmness of closure thus secured is shown by Figure 19, which represents a longitudinal section of the artery, passing through two of the closed segments. The forces which press together the opposed surfaces are equal and opposite, and the parts of the segments which correspond to these surfaces undergo no strain. The lunulæ, therefore, like the mutually opposed portions of the mitral or tricuspid valve, are very delicate and flexible, while the rest of each semilunar segment is strongly made, to resist of itself the arterial pressure.

1

Corpora Arantii and their Uses.-At the centre of the free margin of each semilunar segment, just between the ends of the two lunulæ, there is a small thickening, more pronounced in the aorta than in the pulmonary artery, called the "body of Aranzi" (corpus Arantii). This thickening both rises above the edge and projects from the surface between the lunulæ. When the valve is closed, the three corpora Arantii come together and exactly fill a small triangular chink, which otherwise might be left open just in the centre of the cross section of the artery (see Figs. 18, 20).

The foregoing shows that the mechanism of the semilunar valves is no less effective, though far simpler, than that of the mitral and tricuspid. That the latter two should be more complex is natural; for each of them must give free entrance to and prevent regurgitation from a chamber which nearly empties itself, and hence undergoes a very great relative change of volume; while the arterial system is at all times distended and undergoes a change of capacity which is relatively small while receiving a pulse-volume and transmitting it to the capillaries.

I. THE CHANGES IN FORM AND POSITION OF THE BEATING HEART, AND THE CARDIAC IMPULSE.

General Changes in the Heart and Arteries.-During the brief systole of the auricles these diminish in size while the swelling of the ventricles is completed. During the more protracted systole of the ventricles, which immediately follows, these diminish in size while the auricles are swelling and the injected arteries expand and lengthen. During the greater part of the succeeding diastole of the ventricles both these and the auricles are swelling, and all the muscular fibres of the heart are flaccid, up to the moment when a new auricular systole completes the diastolic distention of the ventricles, as above stated. During the ventricular diastole, as the great arteries recoil they shrink and shorten. The changes of size in the beating heart depend entirely. upon the changes in the volume of blood contained in it, and not upon changes in the volume of the muscular walls. The muscular fibres of the heart agree with those found elsewhere in not changing their volume appreciably during contraction, but their form only. The cardiac cycle thus runs its course with 1 Named from Julius Cæsar Aranzi of Bologna, an Italian physician and anatomist, born in 1530.

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