Ilarvey states that he has observed that if the finger is applied to the ventricles in those cases where the action of the auricles continues after the contractions of the ventricles have ceased, a distinct beat is felt in the ventricle at each stroke of the auricle; and Senac, in quoting this, adds (evidently from his own observation) that it is similar to the pulse in the arteries. Senac also states that if an opening be made into the apex of the heart under those circum stances, a jet of blood rushes through it at each stroke of the auricle. Ile, however, admits that the contraction of the auricles in these cases is not sufficient to dilate sensibly the walls of the ventricles, but, of course, very considerable allowance ought to be made for the enfeebled state of the auricles at this stage of the experiment.t In the experiments of Dr. Ilope, Mr. Carlisle, M. Bouillaud, and the Dublin Committee for investigating the cause of the sounds of the heart, the contrac tion of the auricles appeared to be compara tively trifling, and was most apparent in the appendices. From my own observations upon rabbits and dogs I am convinced that the au ricles contract considerably more when the movements of the heart are proceeding in a natural manner, than some of these last expe riments would lead us to believe, and that this contraction is not confined to the appendix, but extends over the whole auricle. When the circulation through the lungs becomes im peded, the right ventricle is then unable to empty itself, and the auricle of the same side (and this is the one that is most generally ob served in such experiments) is consequently impeded in its movements. The auricles do not certainly exert the force or contract to the extent which some have stated, do not ex pel the whole of their contents, and their diastole is comparatively feeble; but that none of the muscular fibres of the auricles are pas sive, but exert a force proportionate to their strength, we have evidence both from expe riment and the effects of disease. In some of those cases where an impediment to the passage of the blood from the auricle to the ventricle exists, all the muscular fibres of the auricles become much increased in thickness and in strength. As the left auricle has natu rally greater difficulties to overcome in pro pelling its blood than the right, so we find that the left auricle is considerably more muscular than the right.* The appendix from its being loose, and supplied by a band of longitudinal fibres drawing it backwards, must enjoy a freer motion than the other parts of the auricle.
Systole and diastole of the ventricles.— When the heart is acting vigorously, the con traction of the ventricles succeeds immediately upon that of the auricles, so that they some times appear continuous; or, in other words, the sudden distention of the ventricles by the blood propelled into them during the systole of the auncles is rapidly followed by the con traction of the ventricles. The systole of the ventricles must occur during the diastole of the auricles. As we are only sensible of the sys tole of the ventricles from external examination during life, the expression systole of the heart is always employed as synonymous with the systole of the ventricles. When the action of the heart is a little less active, an apparent in terval is observable between the completion of the contraction of the auricles and the com mencement of the contraction of the ventricles, —the irritability of the ventricles being at this time somewhat impaired, their contraction does not so quickly follow their sudden distention. The ventricles during their systole are dimi nished in all their dimensions; the apex is drawn upwards to the base and tilted forwards so as to strike the parictes of the thorax be tween the cartilages of the fifth and sixth ribs.}
The parietes of the ventricles at this time are firm and resisting, and present some rugs on their outer surface. Haller* states that though the principal movement of the ventricles du ring their systole is from the apex upwards, yet he has sometimes observed a slight but dis tinct movement from the base downwards. The contraction of the ventricles is performed with great force, and, when vigorous, appears to be accomplished by the simultaneous action of all its fibres ; but at other times, when it has be come enfeebled, it has been observed to com mence at the apex and extend itself upwards.
The diastole of the ventricles consists of two distinct stages. The first, which immediately follows its systole, is sudden, the apex being pushed downwards and apparently passing deeper into the chest, and is occasioned by the return of the heart to its state of rest. The second is also sudden, and attended by a rapid but not very extensive enlargement of the heart in all its dimensions. The parietes of the heart are soft and flaccid, and their external surface smooth during their diastole. The diastole of the heart is performed with con siderable force, so that Pechlin, Perrault, I lam berger, and others long ago maintained that this equally with the systole is the result of a vital action. This opinion was again revived by Bichat, Dumas, and their followers, and is still introduced by some into the discussions upon the movements of the heart. Before we can admit an opinion of this kind, it would be necessary that very strong evidence be adduced in its favour, as it is at perfect variance with all that we know of the arrange ment of the fibres of the heart, and of the laws of muscular contractility.t OesterreicherT has performed the following experiment, which appears nearly decisive on this point. When a body is placed on the heart of a frog heavy enough to press it flat, but sufficiently small to allow the heart to be observed, it will be seen that the body will be lifted during the contraction of the heart, but that during its extension it will remain flat. From this it appears that the extension of the heart after the contraction is not a muscular act. The diastole of the heart depends then upon two circumstances. 1st, Upon the na tural elasticity of the organ, which it possesses in common with every other muscle, and by which it instantly resumes its state of rest as soon as its contraction has ceased. This, which is usually termed the relaxation of a muscle in whatever part of the body it occurs, must be expected to be more energetic in the heart than in the muscles of voluntary motion, as from the arrangement of its fibres a great part must be more strongly compressed. This occurs during the first of the two stages into which we divided the diastole. 2d, Upon its sudden distention during the contraction of the auricles, when we have every reason to believe that the ventricles are completely passive. This con stitutes the second stage of the diastole. The blood must then pass from the auricles into the ventricles during each diastole at two dis tinct periods of time, corresponding to these two stages. During the first stage, or the re laxation of the ventricles, it flows from the auricles to fill up the vacuum produced in their interior ; while, during the second stage, it is forcibly propelled by the auricles. It would be difficult to estimate the relative proportion of these two quantities of blood. Those who suppose that the contraction of the auricles is feeble must consequently believe that most of the blood passes from the auricles into the ven tricles during the first stage.