A system of practical medicine. By American authors. Vol. 3

by Unknown

Meckel's original theory for open septum and narrowed pulmonary artery was that the defect was primarily in the septum of the ventricles, due to a want of formative energy, and the pulmonary artery closed itself, as do other arteries, from want of use. Meyer showed that a defect of the septum was incapable of causing narrowing of the pulmonary artery, since the exit of blood is easier through the artery, from the form of the right ventricle, than through the open septum; the passage of the blood from right to left is opposed by the blood-mass in the left cavity. Heine also thought the pulmonary-artery narrowing was a secondary defect, but did not think the opening of the septum caused the narrowing. He considered the primary malformation to be a deviation of the septum to the left. The deviation of the partition before its closure brought the aorta within the left cavity, and furnished a free exit for the blood from this chamber shorter and more convenient than through the pulmonary and the ductus arteriosus Botalli to the descending aorta; the pulmonary artery collapsed for want of use, similarly to other foetal blood-courses. Hence, Heine considered that in all cases of open septum and apparent transposition of the aorta which exhibited no evidences of inflammation as a conjectural cause of narrowing or closure of the pulmonary artery the explanation was to be found in a primary deviation of the septum ventriculorum.

The difficulty in Heine's theory lies in showing the mechanism of a deviation of the septum without a primary obstruction of the flow of blood through the pulmonary artery. The hypertrophy of the right ventricle which Heine proposed as an explanation is almost certainly a secondary effect of the obstruction, and therefore cannot be supposed to originate a deviation of the septum; it is doubtful if hypertrophy can be considered as a cause of increased blood-pressure within the cavity of a ventricle under any circumstances, and certainly not as exercising pressure in a direction to cause the supposed deviation of the septum. An open septum without obstruction of the pulmonary orifice, which rarely occurs, does not produce hypertrophy of the right chamber.

The explanation of cases of open septum with obstruction of the pulmonary artery seems entirely satisfactory by Hunter's theory, or by what Kussmaul has named the engorgement theory. But when there is a real transposition of the arteries, the pulmonary placed farther to the left and behind and coming from the left cavity, the aorta in front and to the right and arising from the right or pulmonary chamber, thus changing their relative positions and their orifices exchanging ventricles, the difficulty of explanation becomes great, and the cause of the abnormal relations of the vessels cannot be traced to a simple deviation of the septum ventriculorum.

For the explanation of these cases of complete transposition of the vessels, as well as their transplantation relatively to the ventricles, Rokitansky has traced respectively the development of the two arterial trunks from the common trunk and of the septum ventriculorum. He considers that the partitioning of the arterial trunks is the governing factor in their formation, and that the ventricular septum is arranged in conformity with the septum of arterial trunks. In tracing the development of the circulatory apparatus in man there seems to be no doubt that the heart develops exactly like that of other vertebrates.

The very first rudiment of the heart is a spindle-shaped thickening of the intestinal fibrous layer of the fore part of the alimentary canal. This spindle-shaped formation then becomes a hollow pouch, and separates from the intestinal layer and lies free in the cardiac cavity. The earliest condition yet seen in the human being is that from an embryo of about two weeks (Coste), in which the viscus appeared as a simple tube in the shape of a letter S—the hollow rounded pouch having slightly elongated and bent to this form, and simultaneously turned spirally on an imaginary axis, so that the posterior part of the tube rested on the dorsal surface of the anterior part. The yelk-veins connect at its posterior part, while the arteries form a continuation of its anterior extremity. The spiral turning and curving increase, and simultaneously two shallow indentations appear in the twisted pouch, transversely to its long axis, looking like kinks in a flexible tube. These indentations mark the outline of the three primitive portions of the central organ—viz. the first, with which the veins communicate, represents the future auricles; the next, the ventricles; the third portion, the common arterial trunk (aortic bulb or truncus arteriosus communis). Early in development the first section is the largest, but by the time the S is formed the middle or ventricular portion exceeds in size the auricles and their appendages. So far, the central organ remains a continuous tube, indented transversely in its course at the points which mark its future division; the blood moves through it as through a coiled tube, entering by the veins and passing out by the aortic bulb to the vascular or branchial arches; the venous entrance is posterior, the arterial exit is anterior and is directed toward the future aortic arch. This is the condition at the end of the second week. The future auricles and ventricles now form a common cavity; the indentation between them, called the auricular canal, represents the future auriculo-ventricular orifice. The future fibrous ring forming this orifice is the first to be developed of all the permanent structures of the heart; its infolding to form the two auriculo-ventricular orifices comes early, but at a later date than here spoken of. Its exact method of development is not clearly described.

Between the second and fourth weeks is exhibited an indication of the future most important step in development; this process does not really step forth until the fourth week, although superficial traces of a furrow antedate this time. This step is the division of three sections of the tube into opposite halves, a right or venous, a left or arterial half. This division results in the formation of the future septa between the auricles and between the ventricles, and separates the common arterial trunk (aortic bulb) into the future aorta and pulmonary artery. This partition is spoken of as longitudinal; but it will be seen, if the real lines of growth of the future auricular and ventricular septa are carefully regarded, that the indentations which mark their site are also transverse, as were the primitive ones for division of the auricles from the ventricles. The proximal end of the tube comes in contact with the distal portion by a further bending movement, so that these two ends go to make the left half of the heart; and the middle portion of tube, composed partly of auricle and partly of ventricle, forms the right half of the heart. This secondary indentation, commonly spoken of as longitudinal, is in reality transverse, although, from the more markedly bent condition of the tube which has come about, it does not advance in the same plane as the primitive indentation of the tube. The mechanism of the division of the aortic bulb will be described later.

This secondary indentation, which finally results in the formation of the auricular and ventricular septa, appears earlier in the ventricular cavity, about the fourth week, and later in the auricles, about the eighth week. By about the twelfth week the process of formation for the muscular partitions is completed; the septum ventriculorum normally is gradually built up, and by this time has joined itself, at the base of the heart, to the septum forming itself in the arterial bulb; thus the right and left ventricles are finally separated. The septum in the auricles is also finished in its muscular part, mostly built up from the base and posteriorly toward the roof of the cavity, leaving, however, the foramen to be closed by the membrane some days after birth.

The foetal heart from the fourth week onward becomes more and more rounded in outline, and finally more or less rectangular. The auricular appendages become conspicuous and overhang the ventricles. The future left ventricle appears larger than the right, and the former projects notably leftward and downward. The aortic bulb or common trunk appears to arise wholly from the right ventricle, although the vessel communicates with both cavities, since at this period the cavities are undivided. The furrow which marks the line of the future septum ventriculorum runs to the left of the root of the common trunk; and until at least as late as the sixth week this trunk appears from the exterior to be in connection only with the future right ventricle.

As early as the sixth week, possibly earlier, a distinct furrow is seen on both sides of the common trunk running longitudinally from its root at the ventricle to its first branch (branchial arch). This indentation does not traverse directly to the ventricular furrow; in fact, at this period the ventricular furrow is not conspicuous at the origin of the trunk toward the base of the heart, the septum within not having risen as yet to the base of the ventricles. During the formation of this furrow the common trunk continues its slow partial rotation on its axis; the rotation of the other parts of the cardiac tube has ceased; the segments of the tube have come to a standstill—become, as it were, fixed and adherent to each other, the proximal to the distal end, the anterior surface to the posterior, through the previous bending of the tube on itself.

Within the common trunk Rokitansky has described the changes, as seen in cross-sections, which result in its division into a permanent aorta and pulmonary artery, and also the adaptation of the septum arteriosus trunci to the septum ventriculorum. He says that at an earlier period than here described for the external furrow appearing, on the inner surface of the truncus arteriosus communis (aortic bulb), to its left side and somewhat posteriorly, above the starting-point of the anterior limb of the septum ventriculorum, a little swelling appears, which grows toward the right and slightly forward, so that the common trunk is divided into an anterior rather left-hand, and a posterior right portion, respectively the pulmonary artery and aorta. The growth does not pass in a straight line through the lumen of the common trunk, but so that the forming septum makes a concavity posteriorly toward the aorta, and a convexity anteriorly toward the pulmonary; thus, on cross-section the aorta has the outline of the gibbous moon—the pulmonary, fitting into it, separated by the septum, of a new moon. The septum ventriculorum, as seen starting at the base of the ventricles from the fibrous ring of the auriculo-ventricular orifice (having already been built upward from the future apex of the heart), originates at a point on the posterior wall of the common ventricular cavity in exact correspondence with the starting-point of the little swelling on the inner surface of the common arterial trunk. The two septa are thus formed in apposition. The septum ventriculorum, in advancing forward to meet the other limb of the septum forming on the opposite wall of the ventricular cavity, follows the septum trunci arteriosus communis, surrounds the posterior vessel (the aorta) to its front, then passes around it to its right; the pulmonary is on the other side of the septum; the portion of the septum ventriculorum between the orifices of the vessels is the pars membranacea of the septum. The anterior portion of the septum ventriculorum forms one wall of the arterial conus of the right ventricle. Thus it happens that by the eighth week the common trunk is divided into aorta and pulmonary artery; the structure of the septum ventriculorum is so far advanced that these vascular trunks are connected with the proper ventricles, but the septum ventriculorum does not close completely until about the twelfth week.