I.—The heart, in all stages of its development, is to the vascular system what the point of a circle is to the circumference—namely, at once the beginning and the end. The heart, occupying, it may be said, the centre of the thorax, circulates the blood in the same way, by similar channels, to an equal extent, in equal pace, and at the same period of time, through both sides of the body. In its adult normal condition, the heart presents itself as a double or symmetrical organ. The two hearts, though united and appearing single, are nevertheless, as to their respective cavities, absolutely distinct. Each heart consists again of two compartments—an auricle and a ventricle. The two auricles are similar in structure and form. The two ventricles are similar in the same respects. A septum divides the two auricles, and another—the two ventricles. Between the right auricle and ventricle, forming the right heart, there exists a valvular apparatus (tricuspid), by which these two compartments communicate; and a similar valve (bicuspid) admits of communication between the left auricle and ventricle. The two hearts being distinct, and the main vessels arising from each respectively being distinct likewise, it follows that the capillary peripheries of these vessels form the only channels through which the blood issuing from one heart can enter the other.
II.—As the aorta of the left heart ramifies throughout all parts of the body, and as the countless ramifications of this vessel terminate in an equal number of ramifications of the principal veins of the right heart, it will appear that between the systemic vessels of the two hearts respectively, the capillary anastomotic circulation reigns universal.
III.—The body generally is marked by the median line, from the vertex to the perinaeum, into corresponding halves. All parts excepting the main bloodvessels in the neighbourhood of the heart are naturally divisible by this line into equals. The vessels of each heart, in being distributed to both sides of the body alike, cross each other at the median line, and hence they are inseparable according to this line, unless by section. If the vessels proper to each heart, right and left, ramified alone within the limits of their respective sides of the body, then their capillary anastomosis could only take place along the median line, and here in such case they might be separated by median section into two distinct systems. But as each system is itself double in branching into both sides of the body, the two would be at the same time equally divided by vertical section. From this it will appear that the vessels belonging to each heart form a symmetrical system, corresponding to the sides of the body, and that the capillary anastomosis of these systemic veins and arteries is divisible into two great fields, one situated on either side of the median line, and touching at this line.
IV.—The vessels of the right heart do not communicate at their capillary peripheries, for its veins are systemic, and its arteries are pulmonary. The vessels of the left heart do not anastomose, for its veins are pulmonary, and its arteries are systemic. The arteries of the right and left hearts cannot anastomose, for the former are pulmonary, and the latter are systemic; and neither can the veins of the right and left hearts, for a similar reason. Hence, therefore, there can be, between the vessels of both hearts, but two provinces of anastomosis—viz., that of the lungs, and that of the system. In the lungs, the arteries of the right heart and the veins of the left anastomose. In the body generally (not excepting the lungs), the arteries of the left heart, and the veins of the right, anastomose; and thus in the pulmonary and the systemic circulation, each heart plays an equal part through the medium of its proper vessels. The pulmonary bear to the systemic vessels the same relation as a lesser circle contained within a greater; and the vessels of each heart form the half of each circle, the arteries of the one being opposite the veins of the other.
V.—The two hearts being, by the union of their similar forms, as one organ in regard to place, act, by an agreement of their corresponding functions, as one organ in respect to time. The action of the auricles is synchronous; that of the ventricles is the same; that of the auricles and ventricles is consentaneous; and that of the whole heart is rhythmical, or harmonious—the diastole of the auricles occurring in harmonical time with the systole of the ventricles, and vice versa. By this correlative action of both hearts, the pulmonary and systemic circulations take place synchronously; and the phenomena resulting in both reciprocate and balance each other. In the pulmonary circulation, the blood is aerated, decarbonized, and otherwise depurated; whilst in the systemic circulation, it is carbonized and otherwise deteriorated.
VI.—The circulation through the lungs and the system is carried on through vessels having the following form and relative position, which, as being most usual, is accounted normal. The two brachio-cephalic veins joining at the root of the neck, and the two common iliac veins joining in front of the lumbar vertebrae, form the superior and inferior venae cavae, by which the blood is returned from the upper and lower parts of the body to the right auricle, and thence it enters the right ventricle, by which it is impelled through the pulmonary artery into the two lungs; and from these it is returned (aerated) by the pulmonary veins to the left auricle, which passes it into the left ventricle, and by this it is impelled through the systemic aorta, which branches throughout the body in a similar way to the systemic veins, with which the aortic branches anastomose generally. On viewing together the system of vessels proper to each heart, they will be seen to exhibit in respect to the body a figure in doubly symmetrical arrangement, of which the united hearts form a duplex centre. At this centre, which is the theatre of metamorphosis, the principal abnormal conditions of the bloodvessels appear; and in order to find the signification of these, we must retrace the stages of development.
VII.—From the first appearance of an individualized centre in the vascular area of the human embryo, that centre (punctum saliens) and the vessels immediately connected with it, undergo a phaseal metamorphosis, till such time after birth as they assume their permanent character. In each stage of metamorphosis, the embryo heart and vessels typify the normal condition of the organ in one of the lower classes of animals. The several species of the organ in these classes are parallel to the various stages of change in the human organ. In its earliest condition, the human heart presents the form of a simple canal, similar to that of the lower Invertebrata, the veins being connected with its posterior end, while from its anterior end a single artery emanates. The canal next assumes a bent shape, and the vessels of both its ends become thereby approximated. The canal now being folded upon itself in heart-shape, next becomes constricted in situations, marking out the future auricle and ventricle and arterial bulb, which still communicate with each other. From the artery are given off on either side symmetrically five branches (branchial arches), which arch laterally from before, outwards and backwards, and unite in front of the vertebrae, forming the future descending aorta. In this condition, the human heart and vessels resemble the Piscean pipe. The next changes which take place consist in the gradual subdivision, by means of septa, of the auricle and ventricle respectively into two cavities. On the separation of the single auricle into two, while the ventricle as yet remains single, the heart presents that condition which is proper to the Reptilian class. The interauricular and interventricular septa, by gradual development from without inwards, at length meet and coalesce, thereby dividing the two cavities into four—two auricles and two ventricles—a condition proper to the Avian and Mammalian classes generally. In the centre of the interauricular septum of the human heart, an aperture (foramen ovale) is left as being necessary to the foetal circulation. While the septa are being completed, the arterial bulb also becomes divided by a partition formed in its interior in such a manner as to adjust the two resulting arteries, the one in connexion with the right, the other with the left ventricle. The right ventricular artery (pulmonary aorta) so formed, has assigned to it the fifth (posterior) opposite pair of arches, and of these the right one remaining pervious to the point where it gives off the right pulmonary branch, becomes obliterated beyond this point to that where it joins the descending aorta, while the left arch remains pervious during foetal life, as the ductus arteriosus still communicating with the descending aorta, and giving off at its middle the left pulmonary branch. The left ventricular artery (systemic aorta) is formed of the fourth arch of the left side, while the opposite arch (fourth right) is altogether obliterated. The third and second arches remain pervious on both sides, afterwards to become the right and left brachio-cephalic arteries. The first pair of arches, if not converted into the vertebral arteries, or the thyroid axes, are altogether metamorphosed. By these changes the heart and primary arteries assume the character in which they usually present themselves at birth, and in all probability the primary veins corresponded in form, number, and distribution with the arterial vessels, and underwent, at the same time, a similar mode of metamorphosis. One point in respect to the original symmetrical character of the primary veins is demonstrable—namely, that in front of the aortic branches the right and left brachio-cephalic veins, after joining by a cross branch, descend separately on either side of the heart, and enter (as two superior venae cavae) the right auricle by distinct orifices. In some of the lower animals, this double condition of the superior veins is constant, but in the human species the left vein below the cross branch (left brachio-cephalic) becomes obliterated, whilst the right vein (vena cava superior) receives the two brachio-cephalic veins, and in this condition remains throughout life. After birth, on the commencement of respiration, the foramen ovale of the interauricular septum closes, and the ductus arteriosus becomes impervious. This completes the stages of metamorphosis, and changes the course of the simple foetal circulation to one of a more complex order—viz., the systemic-pulmonary characteristic of the normal state in the adult body.
VIII.—Such being the phases of metamorphosis of the primary (branchial) arches which yield the vessels in their normal adult condition, we obtain in this history an explanation of the signification not only of such of their anomalies as are on record, but of such also as are potential in the law of development; a few of them will suffice to illustrate the meaning of the whole number:—lst, The interventricular as well as the interauricular septum may be arrested in growth, leaving an aperture in the centre of each; the former condition is natural to the human foetus, the latter to the reptilian class, while both would be abnormal in the human adult. 2nd. The heart may be cleft at its apex in the situation of the interventricular septum—a condition natural to the Dugong, A similar cleavage may divide the base of the heart in the situation of the interauricular septum. 3rd. The partitioning of the bulbus arteriosus may occur in such a manner as to assign to the two aortae a relative position, the reverse of that which they normally occupy—the pulmonary aorta springing from the left ventricle and the systemic aorta arising from the right, and giving off from its arch the primary branches in the usual order. [Footnote 1] 4th. As the two aortae result from a division of the common primary vessel (bulbus arteriosus), an arrest in the growth of the partition would leave them still as one vessel, which (supposing the ventricular septum remained also incomplete) would then arise from a single ventricle. 5th. The ductus arteriosus may remain pervious, and while co-existing with the proper aortic arch, two arches would then appear on the left side. 6th. The systemic normal aortic arch may be obliterated as far up as the innominate branch, and while the ductus arteriosus remains pervious, and leading from the pulmonary artery to the descending part of the aortic arch, this vessel would then present the appearance of a branch ascending from the left side and giving off the brachio-cephalic arteries. The right ventricular artery would then, through the medium of the ductus arteriosus, supply both the lungs and the system. Such a state of the vessels would require (in order that the circulation of a mixed blood might be carried on) that the two ventricles freely communicate. 7th. If the fourth arch of the right side remained pervious opposite the proper aortic arch, there would exist two aortic arches placed symmetrically, one on either side of the vertebral column, and, joining below, would include in their circle the trachea and oesophagus. 8th. If the fifth arch of the right side remained pervious opposite the open ductus arteriosus, both vessels would present a similar arrangement, as two symmetrical ducti arteriosi co-existing with symmetrical aortic arches. 9th. If the vessels appeared co-existing in the two conditions last mentioned, they would represent four aortic arches, two on either side of the vertebral column. 10th. If the fourth right arch, instead of the fourth left (aorta), remained pervious, the systemic aortic arch would then be turned to the right side of the vertebral column, and have the trachea and oesophagus on its left. 11th. When the bulbus arteriosus divides itself into three parts, the two lateral parts, in becoming connected with the left ventricle, will represent a double ascending systemic aorta, and having the pulmonary artery passing between them to the lungs. 12th. When of the two original superior venae cavae the right one instead of the left suffers metamorphosis, the vena cava superior will then appear on the left side of the normal aortic arch. [Footnote 2] Of these malformations, some are rather frequently met with, others very seldom, and others cannot exist compatible with life after birth. Those which involve a more or less imperfect discharge of the blood-aerating functions of the lungs, are in those degrees more or less fatal, and thus nature aborting as to the fitness of her creation, cancels it.
[Footnote 1: This physiological truth has, I find, been applied by Dr. R. Quain to the explanation of a numerous class of malformations connected with the origins of the great vessels from the heart, and of their primary branches. See The Lancet, vol. I. 1842.]
[Footnote 2: For an analysis of the occasional peculiarities of these primary veins in the human subject, see an able and original monograph in the Philosophical Transactions, Part 1., 1850, entitled, “On the Development of the Great Anterior Veins in Man and Mammalia.” By John Marshall, F.R.C.S., &c. ]