The motions of any part of the heart, considered singly and individually, consist in a constant series of alternate contrac tions and dilatations ; or, as they are tech nically named, alternate states of systole and diastole. The contractions take place as in other muscles, the dilating cause consists in the forcible entrance of blood into the cavity. The auricles and ventri cles, when viewed in relation to each other, are successively contracted and di lated; the corresponding parts acting at the same time on both sides of the heart. Thus, when the auricles contract, in or der to expel the blood which they have received from the system at large, and from the lungs, the ventricles are relaxed, and therefore in a state fit for receiving this blood. When, in the following mo ment, the recently filled ventricles con tract, in order to urge terwards the blood into the two arterial trunks, the auricles are relaxed, and become immediately dis tended by the current of venous blood.
The action of the heart, and of the ves sels connected with it, may therefore be distributed into successive periods. In the first of these, the venx caves and pul monary veins pour their blood into the two auricle; and thus cause a diastole of these cavities. The systole of the auricles transmits the blood into the ventricles in the second period ; and these latter ca vities expel their contents into the arteries in the third portion of time. Thus the ac tion of the veins takes place at the same point of time with that the ventricles ; and the contraction of the auricles is syn chronous with that of the arteries.
The systole of the ventricles, which is supposed to occupy one third of the time of the whole pulsation of the heart, is ac complished by an approximation of the sides of the cavities to the middle parti tion, and of the apex to the basis of the heart. The whole viscus by this means becomes shorter and more obtuse. The well known fact of the heart striking against the left breast in its contraction, may seem on the first glance to refute this account of the systole of the ventri cles. But, on a further examination, it can have no such effect ; since the phe nomenon in question depends on two causes amply sufficient to proc4uce the effect. The swelling of the auricles, which are at the back of the heart, and particularly of the left auricle, which is interposed between the spine and the base of the left ventricle, necessarily causes the point of the heart to advance towards the side ; and this motion may be imitat ed in the dead body by injecting or in flating the muscles. The other cause consists in the connection of the large arteries, particularly of the aorta, with the base of the heart. A curved and flexi ble tube, when suddenly distended, be comes in some measure straightened.
Thus, when the blood is impelled into the aorta, the curve of that vessel approach es more nearly to a straight line. Its posterior end being firmly attached to the vertebra is immovable ; to its anterior and moveable part is fixed the heart, which, by the straightening of the vessel, is obliged to describe a portion of a cir cle, in doing which, the apex strikes against the side. These two circumstan ces occur simultaneously ; the venous blood rushes into the auricles at the same time that the ventricle fills the aorta. The impulse of the blood expelled by the aortic ventricle is felt in the whole arte. rial system ; and it produces in all arte ries, which come within the sphere of the touch, and which have an area of not less than one-sixth of a line in diameter, an obvious and perceptible eflect, called the pulse, which is a real state of diastole of the artery, and which is ascertained to correspond exactly, and to be perfectly synchronous with the systole of the heart. The number of pulsations in a given space. of time varies infinitely in healthy per sons. Age is the chief cause of these varieties; but other circumstances, which. constitute the peculiar state of health of each individual, have considerable effect a so that no standard can be settled which shall prove generally correct. The fol lowing numbers afford, we believe, as near an approximation as can be expect. ed amidst so much uncertainty ; they will serve at least as a comparative view in subjects of different ages. The heart of an infant, sleeping tranquilly, performs in the first days of existence about 140 pul• sations in a minute ; at the end of the first year the pulsations are, in the same space of time, 124.
At the end of the second year 110 Third and following years 0 Seventh and following 86 Time of puberty 80 Manhood 75 Sixtieth year 65 The pulsations of the heart proceed in, a regular and continued succession to the last period of life, and then all its parts do not to act at once ; but the right auricle and ventricle survive the opposite cavities for some little time, so that the former part has been called the oltimum moriens. The blood, which re turns by the venx cavx after the last ex piration, no longer finds the usual pas sage through the lungs, which are con tracted, but it is still urged on from be hind by that which the aorta has recently propelled. Hence it is forced into the right auricle, and excites contraction in that part, by the stimulus of its presence, some time after the left side has been at rest. This congestion on the right side of the heart in the last agony explains the empty state of the arteries, particularly the larger ones, after death.