soon as the auricles are full their walls suddenly contract and press the blood into the right and left ventricles, upon which the ventricles immediately contract, and force it into the great

arteries.

The contraction of each pair of cavities is followed by their relaxation.

The blood cannot pass back into the veins from the auricles when they contract, because the auricular contraction com

The Orifices of the Heart seen from above, both the auricles and the great vessels being removed. (Huxley.)

PA. Pulmonary artery and its semilunar valves. Ao. Aorta and its valves.
RAV. Tricuspid, and LA V. Bicuspid valves.

mences in the bundles of muscular fibre which surround the orifices of the great venous trunks; and it cannot flow back to the auricles, because, as has been seen, the force of the blood current on its entry into the ventricles closes the valves; while a backward flow from the large arteries is at once prevented by the current distending the semilunar pockets, and thus firmly closing the valves.

When viewed for the first time, the beat of the heart appears

to be a single act, so rapidly does the ventricular follow the auricular beat. More careful examination shows that this single action is composed of different phases of activity and repose, which together make up the cycle of the heart beat. The contraction of the cavities of the heart is called their systole, the period of rest is called their diastole.

Systole of the Heart.-The systole of the corresponding cavities of both sides of the heart is exactly synchronous; that is to say, the two auricles contract simultaneously, and the contraction of the two ventricles

follows immediately that of the auricles.

The ventricular systole follows that of the auricle so closely that no interval can be appreciated. The rapidly succeeding acts of auricular and ventricular systole are followed by a period during which both auricles and ventricles are in diastole, which is commonly spoken of as the passive interval or pause.

While the auricles are contracting the ventricles are relaxed, and the relaxation of the auricles commences immediately the ventricular contraction begins.

The entire cycle of the heart beat, occupying nearly a second in the healthy

adult, may be divided into three stages:

Auricular systole.

Ventricular systole.

General diastole.

The exact time occupied by each phase of the cycle can only be calculated approximately. This may be done either by registering graphically the motions of the auricles and ventricles directly communicated to levers brought into contact with their surface, or by recording graphically the pressure changes which occur within the cavities, by introducing into them little elastic sacks filled with air, whence the pressure changes are communicated to an ordinary "tambour," and registered on a smoked surface.

Of the whole period of the cycle the passive interval or pause is the longest and the most variable, for in the ordinary changes in the heart's rhythm the pause alone varies. Next in duration is the ventricular systole, while the shortest is the auricular systole.

The following figures give approximately the proportion of time occupied by each part of the cycle in the case of a horse, whose intra-cardiac tension was registered in the manner just referred to while his heart beat about fifty times in the minute:

Or if we assume the human heart to beat some seventy times a minute, each cycle would occupy about of a second, made up as follows:

:

The duration of the auricular and ventricular systole varies little except under abnormal circumstances, but the pause is constantly undergoing slight changes. In fact, the duration of the general diastole depends upon the rate of the heart beat, being less in proportion as the heart beats more quickly.

CARDIAC MOVEMENTS.

If the thorax of a recently killed frog be opened, the heart can be observed beating in situ, and the different acts in the cycle studied without difficulty.

In mammalians, in order to see the heart in operation, it is necessary to keep up artificial respiration, during which the heart continues to beat regularly, though the thorax be opened. A careful inspection of the beating heart shows that during its cycle of action certain changes take place in the shape and relative position of its cavities. This is owing partly to the change in the amount of their blood contents and partly to the form assumed by the muscular wall when contracting.

During the passive interval the auricles are seen to swell gradually on account of the blood flowing into them from the veins : when the auricular cavities are nearly full, a contraction, commencing in the great venous trunks near the heart, passes with increasing force over the auricles and gives rise to their rapid systolic spasm. The auricles suddenly diminish in size, and appear to become pale. When the blood is being propelled through the auriculo-ventricular openings, the flaccid walls of the ventricles appear to be drawn over the liquid mass by the contraction of the muscular walls of the auricles (just as a stocking is drawn over the foot by the hands), and the base of the ventricles is thus drawn upward. The moment the ventricles have received their full charge of blood from the auricles they contract, becoming shorter by the movement of the base toward the apex, and thicker by their elongated cone becoming rounder. The great arteries are at the same time distended with the blood from the ventricle and elongated, their elastic walls being drawn down over the liquid wedge. The soft elastic tissues are thus in turn made to slide, as it were, over the incompressible fluid that forms the fulcrum, which the muscular walls use as a purchase.

During the systole, when the thorax is open, the ventricles rotate slightly on their long axis, so that the left comes a little forward, and the apex also forward and toward the right. When the systole of the ventricles ceases, they become flaccid and flattened, and the gradual refilling of the cavities begins, as there is nothing to prevent the blood flowing from the veins through the auricles into the ventricles, where the pressure, as in all parts of the thorax, is negative. The semilunar valves being closed, the large arteries grasp firmly the blood, and by their

steady resilient pressure force it on toward the distal vessels. During this pause the arteries seem to become shorter and to draw the base of the heart up again by lengthening the flaccid ventricles.

The part of the heart which changes its position most is the line between the auricles and ventricles, while the apex remains fixed in one position, only making a very slight lateral and forward motion, which probably does not take place within the thorax. If a thin needle with a straw attached be made to enter the apex through the wall of the chest, the straw does not move in any definite direction during the systole, but simply shakes.

Cardiac Tambour, which can be strapped on to chest wall, so that the central button lies over the heart beat, and the pressure may be regulated by the screws at the side. To the tube bent at right angles is attached the rubber tube which connects the air cavity with that of the writing tambour shown in Fig. 119.

If, on the other hand, the needle be placed in the base of the ventricles, the straw moves up and down with each systole and diastole.

HEART'S IMPULSE.

The heart communicates its motion to the chest wall, and the movement can be felt and seen over a limited area, which varies with the thinness of the individual. This cardiac impulse, as the stroke is called, can best be felt in the fifth intercostal space, a little to the median side of the left nipple. It is found to be synchronous with the ventricular systole. During this period