Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/116

Rh 100 VASCULAR SYSTEM slowly relaxes. (3) The time of the contraction of the auricle and that of its relaxation are about equal, tint the time of the relaxation of the ventricle is nearly twice as long as that of its contraction ; the movements of the auricle are thus uniform and wave-like, whilst those of the ventricle have more of a spasmodic character. (4) The auricular movement (ab, a b, a&quot;b&quot;, g, g , g&quot;) precedes the ventricular, and the latter coincides with the impulse of the apex against the wall of the chest, as is seen from the second vertical dotted line. (5) The contraction of the auricle, by forcing blood on wards, affects the pressure for an instant in the ventricle, as is indi cated by the little elevation immediately before the ventricular con traction. (6) During the period of contraction of the ventricle there are oscillations of pressure affecting both the auricle and the ventricle ; these are indicated by the little waves d, e, f, d, e , f, d&quot;, c&quot;, and/&quot; ; similar waves are seen at h, i, h, i , h&quot;, and i&quot;. The letters k, e, k, e , and k&quot;, e&quot; represent a third set of waves. Sketch of With these facts in view, we may now describe the phenomena cardiac which happen in a complete cardiac revolution. Suppose the blood revolu- to be pouring from the venae cavte and the pulmonary veins into the tion. two auricles. At that moment the auricles are passing into a state of complete diastole, and their cavity is increased by the funnel-shaped aperture at the auriculo- ventricular openings formed by the segments of the valves guarding these orifices. The dis tension of the auricles is due partly to the pressure in the venae cavse and pulmonary veins being less than in the interior of the auricles and partly to the aspirating action of the thorax during inspiration, sucking, as it were, the blood from the veins outside the chest to those inside the chest, and thus favouring the flow of blood to the heart. During this time both ventricles are filling with blood, the auriculo-ventricular orifices being open. When the distension of the auricles is complete (which happens before the distension of the ventricles, because the capacity of the auricles is much smaller than that of the ventricles), the auricular systole commences by the contraction and emptying of the auricular ap pendix towards the general cavity of the auricle, and by the mouths of the veins becoming narrowed by contraction of the circular fibres in their coats. These rhythmic movements are propagated quickly over the auricular walls, causing them to contract simultaneously towards the auriculo-ventricular orifices. The contracting wall forces the blood chiefly in the direction of least resistance, that is, into the ventricle, which at the same time is only partially full of blood, and is passing into a state of complete relaxation. The pressure in the veins, aided by their rhythmic contraction at the commencement of the auricular systole, is sufficient to prevent the blood from passing backwards, except to a very slight extent ; but there is a momentary arrest of the flow in the large venous trunks. Thus the auricles act, not only as passive reservoirs for the blood in its passage from the veins to the auricles, but as rhythmic cavities tending to keep up a mean pressure in the veins, in diminishing by their extensibility the pressure which tends to increase during the ventricular systole, and in increasing the pressure by their contrac tion at a time when the venous pressure would diminish, that is, towards the close of the ventricular diastole. Both auricles and ven tricles exercise, during their diastole, a certain aspirating or sucking action, like that seen during the relaxation of a compressed india- rubber bag ; but this force is very feeble in the case of the heart. The amount of blood discharged into the ventricles (already partially filled during the relaxation of the auricles) by the auricular systole is sufficient to fill their cavities, and consequently the ven tricular systole immediately follows the contraction of the auricles. During the inflow of blood from auricles to ventricles the auriculo- ventricular valves are floated upwards into a more or less horizontal position, and the assumption of this position is further aided by the contraction of the longitudinal muscular fibres that pass from the auricles into the cusps of the valves. When the ventricular walls contract, the margins of the auriculo-ventricnlar valves are closely pressed together, and the cusps are kept from being folded backwards into the auricle by the simultaneous contraction of the musculi papillares pulling on the chordae tendinere which are affixed to the ventricular aspect of the valves. The close apposition of the cusps is also increased, even along their margins, by the arrange ment that the chordae tendineae of one papillary muscle always pass to the adjoining edges of two cusps. Thus the valves, tricuspid on the right side and mitral on the left, are tightly closed and the blood cannot regurgitate into the auricles. The blood, thus com pressed, can only pass into the pulmonary artery from the right ventricle and into the aorta from the left. The positive pressure in the ventricles is at its maximum at the beginning of their con traction ; during the contraction it diminishes ; and at the close of the systole (Marey), or in the diastole immediately thereafter (Goltz and Gaule), or even, according to Moens, shortly before the systole has reached its height, the pressure may even become negative. Moens &quot; explains this aspiration as being due to the formation of an empty space in the ventricle caused by the energetic expulsion of the blood through the aorta and pulmonary artery &quot; (Landois and Stirling). As the blood passes from the ventricles into the pul monary artery and aorta, the segments of the sigmoid valves are forced open and stretched across the dilatations or sinuses behind each cusp, without being actually pressed against the walls of the vessels ; and, as both the pulmonary artery and the aorta contained a certain amount of blood before, the pressure in these vessels is in creased, and the walls of both yield to a considerable extent. As already stated, the ventricle continues in the contracted state for a brief space of time, and then it relaxes. Simultaneously with the commencement of relaxation, the auriculo-ventricular orifices open, thus permitting the passage of blood from the auricles ; and at the same time the elastic walls of the aorta and pulmonary arteries recoil and force a portion of the blood backwards towards the cavities of the ventricles, in which, as they are passing into diastole, the pressure is much less than in the vessels. This blood, however, by filling the sinuses of Valsalva and the crescentic pouches of the sigmoid valves, closes these latter, and thus prevents any blood from passing into the ventricles. From the end of the ventricular con traction to the moment when the auricles are again full, all the cavities of the heart are in a condition of dilatation and the cavities are filling with blood. This is the period of the pause, during which the heart may be supposed to be in a state of rest. When one watches an actively beating heart exposed in an anaes thetized animal, the movements are so tumultuous and rapid that the eye can not follow them so as to convey to the mind a correct conception of the rapid changes in form. Owing to this our notions of such changes have been de rived chiefly from an inspection of the heart after death. Recent ingenious in vestigations by Ludwig and Hesse have shown that the post-mortem form is not the natural shape of the living heart Fro. s. Projection of the base either in diastole or in systole, but such in systole and diastole ; RV, as is shown, for example, in fig. 8. W* ventricle ; LV, left ven- The apex beat or shock of the heart is synchronous with the systole, and is caused normally by the apex of the ventricle pressing more firmly against the chest wall, from which it is separated when the heart is at rest by the thin margin of the lung. At the time of ventricular systole, as already seen, the heart, instead of being an oblique cone having an elliptical base, as in rest, becomes more like a regular cone, having a cir cular base. When contraction occurs, the apex is carried from below and behind, upwards and forwards, and is forced into the intercostal space, and at the same time the ventricular portion twists on its long axis from left to right, so as to expose partially the left ventricle. It is the twisting motion that gives the shock or impulse, and it is caused chiefly by the contraction of the oblique fibres in the ventricles which lift up the apex ; it is also assisted by the slightly spiral arrangement of the aorta and pul monary artery. Some have supposed that the movement is partly due to the recoil of the ventricles after discharging their blood (like that of an exploded gun), causing the apex to go in the op posite direction, downwards and outwards ; others have held that the discharge of blood into the pulmonary artery and aorta causes an elongation of these vessels, whereby the apex is pushed down wards and forwards. Both of these mechanisms, however, must have only a slight effect, as the cardiac impulse occurs even when from haemorrhage the pulsating heart is practically empty. To obtain a tracing of the apex beat an instrument termed the cardiograph is employed, various forms of which are figured in Landois and Stirling s Human Physiology, vol. i. p. 89 sq. Figs. Change in shaj of hear Apex beat. Cardio- graphic tracings Fio. 9. Tracing of cardiac pulsations of a healthy man. (Marey.) 9 and 10 are examples of tracings obtained by this instrument. From the latter figure we learn the subjoined information. The time of the pause and the contraction of the auricle are represented by ab, and it is evident that the latter pheno menon causes the apex of the heart to move towards the intercostal space. The portion be corresponds to the contraction of the ventricles and is synchronous with the first sound. The curve then rapidly falls as the ventricles relax, and during the descent there are two eleva tions, d and e, synchronous with the Fio. 10. -Normal curve from man. (Landois.) second sound. As already stated, when the ventricles relax, the blood in the aorta and pulmonary artery, driven backwards by the elastic recoil of the walls of these vessels, closes the semilunar valves.