pleted about the end of the second month. About the same time as this process is perfected a septum between the auricles begins to be formed. It arises from the anterior wall, and grows backwards towards the entrance of the venous sinus, and is not completed during fœtal life, but leaves an opening between the two sides, the foramen ovale, which normally persists till after the birth of the child. From the arterial bulb the main stems of the aorta and pulmonary artery are developed. It is divided into two parts by a septum which is formed about the same time as the septum ventriculorum, and consists of the projection inwards of two lateral folds which unite and separate the bulb into two distinct channels, the one communicating with the right, and the other with the left ventricle. The septum of the bulb begins at the distal part and advances towards the ventricles, so that it and the septum of the ventricles develop towards one another, and if the conditions are normal the two septa become continuous. The diagrams illustrate the changes which take place in the development of the heart and great bloodvesseis. Fig. I represents the heart of the embryo at about the 15th day, the loop a corresponds to the bulbous arteriosus, while v represents the venæ omphalo-mesaraicæ, and the two enlargements (6) the auricles. Between these two loops there is a central portion (c) which is the embryonic ventricle. Fig. II shows diagrammatically the bulbus arteriosus with five pairs of aortic arches. Numbering from above downwards the central portions of the first and second pairs become obliterated, while of the three remaining pairs the third, with the internal and external portions of the first and second, go to form the internal (a) and external (6) carotids. The fourth pair on the right side forms the innominate (c) and a portion of the right subclavian (d); on the left side the arch of the aorta (e) and the left subclavian. Of the fifth or lowermost pair the right (ƒ) becomes obliterated, while the left (g) is developed into the pulmonary artery (h) and ductus arteriosus (2). These are the changes which take place in the arterial system. I have not said anything about the development of the great veins as they have not such a direct bearing on the subject. Diagram III represents the course of the fœtal circulation. The umbilical vein (a) on entering the body of the foetus divides into the following branches: Ist, branches to be distributed to the liver (6); 2nd, the ductus venosus (c) which enters the inferior vena cava (d) after receiving the blood from the portal vein (e). The blood enters the right auricle (ƒ) by the superior vena cava (g), and the inferior vena cava (d). The blood from the former passes almost in its entirety through the tricuspid valve into the right ventricle (), while the stream supplied by the latter is directed by the Eustachian valve through the foramen ovale into the left auricle (k), and thereafter into the left ventricle. We have now traced the course of the blood into the two ventricular cavities. When the ventricles contract, the left propels the blood into the ascending aorta (/), while that contained in the right ventricle becomes divided into two streams-one portion of which is transmitted through the pulmonary artery (m) to the lungs, the other, probably the larger, passes through the ductus arteriosus (0) into the descending aorta (n) and is distributed to the lower extremities. The changes which occur at birth on the establishment of respiration, are—An increase in the quantity of blood transmitted to the lungs; the ductus arteriosus and ductus venosus become impervious, and the foramen ovale is closed by its valve as soon as the pressure in the left auricle is equal to that in the right. The development of the heart is now complete. It not unfrequently happens, however, that the changes which I have described are arrested, and malformations of the heart or great vessels are the result. The character of these malformations depends upon the period of intra-uterine life at which the arrest of developement occurs. For example, if the arrest happens about the sixth or eighth week, and should the separation of the left subclavian from the left carotid be retarded, then the two vessels will spring from a common trunk, and you will have a left innominate artery. Or again, should the septum in the bulbus arteriosus not descend so as to unite and form a continuous partition with the septum ventriculorum, then you may have either a transposition of the aorta and pulmonary artery, or both vessels may arise from either the right or left ventricles. These are examples of malformations occurring from arrest in development during the early life of the embryo; there are others which are normal conditions during fœtal life, but must be regarded as abnormal if they persist after birth; as, for example, permanent patency of the ductus arteriosus or of the foramen ovale. We have, therefore, by considering the period at which the development of the parts becomes perverted or arrested, a means of classifying malformations. This arrangement has been adopted by Dr. Peacock. He says: "The cardiac anomalies are classified into: 1. Those dependent upon arrest of development at an early period of fœtal life, or probably from about the fourth to the sixth week, so that the organ retains its most rudi mentary forms-the auricular and ventricular cavities being still single, or presenting very slight indications of division, and the primitive arterial trunk being retained, or the aorta and pulmonary artery very imperfectly evolved. b. Irregularities in the valves. c. Disproportion of the vessels, orifices, and cavities III. Malformations occurring after birth. accompanied by Cyanosis. I. Alterations in size or form of cavities. a. Dilatation and hypertrophy of right chambers of the heart. b. Contracted state of left chambers. II. Lesions of the orifices. III. a. Obstruction of pulmonary artery from new formations or degenerative changes, or from closure by membranous septa. Diseases of the great vessels. a. Constriction of pulmonary artery. b. Dilatation, or aneurisms of the aorta. 2. Those in which the arrest of development Lesions of the Heart and Great Vessels which may be occurs at a more advanced period of fœtal existence, or probably from about the sixth to the twelfth week, when the auricular and ventricular partitions have already been partly formed, and the aorta and pulmonary artery more or less completely developed. Such are the cases in which, with imperfect separation of the ventricles, the arterial or auriculo-ventricular apertures are constricted or obliterated, and the origins of the primary vessels, and especially of the aorta, are misplaced. 3. Cases in which the development of the heart has progressed regularly till the later periods of fœtal life, so that the auricular and ventricular septa are completely formed, and the primary vessels possess their natural connections; but in which, from the occurrence of disease, the organ is either prevented from undergoing the changes which should ensue after birth, or there are slighter morbid conditions, which may become the source of serious obstruction at more advanced periods of life. Such are the premature closure of the fœtal passages, and the occurrence of irregularities in the number and form of the valves." The irregularities of the primary vessels may be similarly classed into: 1. Those taking place at the earlier periods of foetal life, and consisting in the defective evolution of the aorta and pulmonary artery from the primitive vessel and branchial arches. 2. Those in which the development of the aorta and pulmonary artery is less deranged, but in which there are defects which may give rise to serious results in after life. The former class includes the cases in which the origins of the pulmonary artery and aorta are transposed, or the descending aorta is wholly or in part derived from the pulmonary artery. Of the latter class, the most striking examples are the cases in which the aorta beyond the origin of the left sub-clavian artery is more or less constricted, and so occasions disease which may ultimately lead to very marked contraction, or even entire obliteration of the canal." Table I contains a list of the malformations of the heart and great vessels which may produce cyanosis, and in Table II, the lesions resulting from diseased processes, which may be accompanied by the same series of symptoms, are enumerated. Let us now consider some of these in detail. For our present purpose the first two divisions, although of considerable interest, are not of great importance, because, in the great majority of cases the subjects of them die, if not before, more usually shortly after birth. Some cases, however, have been recorded where children, in which the foramen ovale was widely open, and the septum of the ventricles only rudimentary, have lived for a considerable number of weeks. The third and fourth divisions should be considered together-namely, the cases where the heart is divided into four cavities, but where the septum ventriculorum remains imperfect. This condition is frequently associated with one or other of the following malformations:-obstruction, constriction, or incomplete development of the great vessels, or transposition of the aorta and pulmonary artery. The question which now presents itself is, why are these malformations associated, and how far do they account for the symptoms which we include in the term cyanosis? When speaking of the development of the heart, I pointed out that the septum in the bulb was formed at the same time as the septum ventriculorum-namely, about the seventh week. The septum of the ventricles arises from the anterior and lower border of the right wall and grows towards the base of the heart, and, under normal circumstances, ultimately becomes united to the base between the two auriculo-ventricular orifices, and with the septum of the arterial bulb. The last part of the base is that situated at the root of the aortic septum ventriculorum which unites with the bulb, and it is just at this point that abnormal communications between the right and left ventricles are most apt to take place. Figs. IV and V are examples of this malformation. |