It has been long disputed whether or not the ventricles empty themselves completely during each systole. It is very difficult to perceive anything like correct data upon this point in the warm-blooded animals with opaque hearts; but reasoning from analogy, from what we see in the cold-blooded animals whose hearts be come quite pale during each systole, (not, as Harvey supposed, from the blood being pressed out of its parietes, but from the blood in its cavity, seen through its transparent sides, being almost entirely expelled during its systole,) we would be inclined to believe that little blood remained after each systole in the active state of the organ, while we can easily sup pose that a greater or less quantity is left after each contraction when the organ is less vi gorous.
It was the subject of a violent dispute at the commencement of the last century between the Montpellier and Parisian anatomists and phy siologists, whether or not the heart became shortened or elongated during its contraction. In all the warns-blooded animals at least it undoubtedly becomes shortened.* We may at the same time state that the obliteration of the cavity of the ventricle depends much more upon the approximation of its sides than the drawing up of the apex.
Impulse of the heart.—It has been at various times, and still is by some late and modern experimenters,-I- maintained that the apex of the heart strikes the parietes of the thorax during its diastole, and not during its systole. This is in reality what we would (I priori expect, for it certainly does at first appear somewhat paradoxical that the heart should strike the parietes of the chest when the apex is ap proximated to the base. The concurrent tes timony of the most accurate observers has, however, fully established the correctness of the fact. Harvey observed it in the human body when the heart had been exposed from the effects of One of the principal arguments adduced in support of this opinion by these authors was drawn from the fact that the pulse at the wrist is not synchronous with the impulse against the chest, an opinion which had been pretty generally maintained since the time of Aristotle. It is difficult to be convinced of this when the pulse is quick ; but when it is slow, and in certain cases of disease of the heart, it can generally be satis factorily ascertained. So far then they are right, but in the next and most important step of the argument they fall into a decided error; for they proceed upon the supposition that the pulse is synchronous in all the arteries of the body at the same time, and consequently the impulse of the heart at the chest cannot be synchronous with the flow of blood along the arteries, or, in other words, with the systole of the heart. In opposition to this opinion, Dr. Young-l- had previously shown upon the principles of hydraulics that the pulse along the arteries must be progressive, yet in general so rapid as to appear to arrive at the extremities of the body without the intervention of any perceptible interval of time. And when the attention of medical men was turned to this subject, various observers soon ascertained by repeated experiments that the pulse could be felt in favourable cases to pass along the arteries in a progressive manner, — that the pulse in the large arteries at the root of the neck and impulse at the chest are synchronous or nearly so, that both precede that at the wrist, and more distinctly still that of the dorsal artery of the foot.t Various attempts have been made to explain in what manner the apex of the heart is made to impinge against the parietes of the chest by those who maintain that it occurs during the systole of the ventricles. Senac supposed that this was principally effected by the curvature of the two large arteries, but principally of the aorta, which arise from the ventricles; for at each stroke of the ventricles when an addi tional quantity of blood is driven into the large arteries, as they are curved they make an at tempt to straighten themselves; and as this takes place to a slight extent, the heart, which is attached to their extremities, ought to be displaced, and its apex, which describes the arc of a circle greater than the other parts of the heart, is thus made to impinge against the walls of the chest. Ile also believed that the distention of the left auricle with blood during its diastole has also, from its position between the spine and base of the heart, the effect of pushing the heart forwards; and this occurring at the same time with the attempt which the curved arteries make to straighten themselves, it thus acts as a second or subsidiary cause in tilting the heart forwards.* Though this sup posed effect of the curvature of the large arteries has been a favourite explanation with many of the impulse of the heart against the chest, yet it really appears to have little, if any, influence in producing this. Shcbeare,-1- and, more lately, Dr. Corrigan,t have shown that the direction of the curvature of the large ar teries is such, that if any effect of this kind is produced, the heart would not be carried to the left side, but in the direction of the curve, which is exactly in the opposite direction. Besides the tilting forwards of the heart has been observed though no blood was passing along the large vessels at the time, and the same thing takes place after the large vessels have been cut through and the heart removed from the body.§ Haller and others have sup posed that the secondary cause assigned by Senac,—viz. the sudden distention with blood of the left sinus venosus which lies impacted between the spine and left ventricle,—is the principal if not the sole cause by which the heart is pushed forwards against the ribs. In confirmation of this opinion llaller states II that if we inflate the left auricle after having opened the chest, we see the point of the heart approach with vivacity the region of the mam ma. As we cannot, however, under these cir cumstances distend the auricle without also distending the corresponding ventricle, this movement of the heart depends more upon the sudden inflation of the ventricle than upon any distention of the auricle, as any one may easily satisfy himself by repeating the experiment. Besides, the distention of the auricles by the blood flowing along the veins is too gradual for this sudden and rapid impulse of the heart ; nay more,—the impulse may be observed when no blood is flowing into the auricles. Sabatier' believed that this impulse depends upon two causes,—ist, principally upon the distention of the auricles, more particularly the left ; and, 2dly, upon the curvature of the large arteries. Apparently, however, perceiving the necessity of there being a sudden distention of the auri cles to produce this, he supposed that this was effected by the auriculo-ventricular valves. Ile argued that, as these valves during the diastole of the heart form a cone stretching from the base towards the point of the ventricle, which is full of blood when the systole commences, when the valves are carried upwards to ob struct the auriculo -ventricular orifices, this blood is pushed before them into the auricles, producing a reflux into the auricles, which, with the blood flowing along the cavm and pul monary veins, causes a sudden distention of the auricles, which pushes the ventricle forwards4 Meckel appears to have adopted the opinions of Sabatier. We need not repeat our objec
tions to this explanation. Dr. Alison, per ceiving the insufficiency of all these explana tions, has for a considerable time past sug gested in his lectures, that this might be ex plained by the arrangement of the fibres, " more particularly by the irregular cone which they form, being flattened posteriorly, and by the consequent greater mass of fibres on the anterior surface." More lately Mr. Carlisle I has also attempted to explain this by the greater length of the anterior fibres of the heart than of the posterior. As the shape of the ventricles is an oblique cone, and as they have their long est sides in front, he argues, " that it is a law of muscular contraction that fibres are shortened during their contraction in proportion to their length when relaxed. For instance, if a fibre one inch long lose by contraction one fourth of its length, or one quarter of an inch, a fibre two inches in length will lose one inch by contractions of equal intensity. The apex then does not approach the base in the line of the axis of the ventricles, but is drawn more to the side of the longer fibres, that is, towards the front, thus producing the tilting forwards." We believe that it may be proved on mechani cal principles, that though the anterior and left surfaces of the ventricles are considerably longer than those on the posterior and right, yet during their contraction, when they are drawn towards their fixed attachments, if the fibres are of equal thickness, the apex will be drawn up nearly in the diagonal of the two forces, and that if any tilting upwards of the apex take place, this will be only to a small extent, and be quite insufficient to account for the impulse felt at the chest. We must therefore look to some other circumstances besides a mere diffe rence in length of the two surfaces to account for this. Mr. Alderson* has ingeniously at tempted to apply the law of action and reaction between bodies,—one of considerable import ance in mechanical philosophy, and upon which Barker's centrifugal mill has been constructed. Unfortunately, however, for this explanation, the axes of the large arteries and the direction in which the apex is tilted do not by any means accord. Dr. Hope's supposition that "the re tropulsion of the auricular valves" may assist in producing this impulse, " as these act on a column of blood which offers a greater resist ance than the weight of the heart, the action is reflected on the organ itself and impels it for wards," is, on the other band, completely op posed to the law that action and reaction are the same. As well may a man attempt to pro pel a boat by standing in the stern, and push with an oar against the prow. Dr. Filhos attri buted the impulse to the spiral turns of the fibres at the apex of the heart attempting to straighten themselves during their contraction, and so raise themselves suddenly and throw themselves forwards. The objections to this explanation are so palpable that they must occur to every one. Since the tilting of the apex of the heart forwards is observed after the blood has ceased to flow through its cavities, it is obvious that we must look for the cause of this in the arrangement of the muscular fibres themselves, though it may be difficult to point out that particular arrangement. It appears to me that the distribution of some of the strong bands of fibres, the course of which I have already described when treating of the muscu lar tissue of the heart, may satisfactorily account for it. We there pointed out that several strong bands of fibres arise from the base of the septum between the ventricles, pass downwards and form part of the septum, then emerge from the anterior longitudinal groove (fig. 274, d), and wind round in a spiral manner to form both the anterior and posterior part of the lower portion of the heart. On entering the apices of the ventricles, (principally the left,) the fibres are scattered over their inner surfaces, and while a great number of them go directly to be inserted into the tendinous rings, others form part of the column carnew. We have thus strong bands of fibres attached by one extremity (their septa) extremity) to the base of the ventricles at a point pretty far posterior, while at the other ex tremity many of the fibres are loose, or at least only attached to the tendinous rings through the media of the chordw tendineee and valves, which must admit of a certain degree of con traction of these fibres before they become tense. At each systole of the heart when these fibres act, it is evident that the tendinous rings must form the fixed points towards which all these fibres contract ; and since they are by one ex tremity all closely and directly connected to a fixed attachment, viz. the tendinous rings, while by their other extremity part only are directly attached to the tendinous rings, the other part being loose, or at least only connected to the tendinous rings through the lax chordal tendi new and valves, it must follow that the force with which the contraction takes place towards the septa) extremity must preponderate over the other. these bands of fibres had been as closely connected to the tendinous rings at the one extremity as at the other, then the force of the contraction towards both would have been equal ; but since this is not the case, the apex must be carried forwards at the same time that it is drawn upwards towards the base. This forward motion may also probably be assisted by another arrangement of the same fibres which we have been describing; for some of these muscular bands are attached by their inner extremity to the anterior part of the left auriculo-tendinous ring, so as to form loops, the greater part of which lie more in front than be hind the axis of the heart, and may have a ten dency, when in a state of contraction, to draw the apex forwards and upwards. Now when we remember that by this elevation of the apex forwards, the heart, before placed obliquely, now becomes more horizontal, and conse quently more approximated to the walls of the chest,—the more particularly as the transverse diameter of the chest diminishes rapidly as we proceed from below upwards, we believe that we have here sufficient to account for this im pulse against the chest. As the proximity of the apex of the heart to the chest is affected by the position of the body, as we have already pointed out, this circumstance ought to be at tended to in judging of the strength of the im pulse of the heart.