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The nerve supply and conducting system of the human heart at the end of the embryonic period proper

ERNEST GARDNERŸ AND RONAN O’RAHILLY

+tDepartments of Neurology, Orthopaedic Surgery, and Human Anatomy, School of Medicine, University of California, Davis, and Carnegie Institution of Washington, Department of Embryology, Davis Division, Davis, California 95616

Department of Human Anatomy, School of Medicine, University of California, Davis, and Carnegie Institution of Washington, Department of Embryology, Davis Division, Davis, California 95616

J. Anat. (1976), 121, 3, pp. 571-587 571 With 26 figures

(Accepted 24 July 1975)

INTRODUCTION

The availability of a Carnegie Stage 23 human embryo of exceptional histological quality provided an opportunity to study the conducting system at the end of the embryonic period proper (8 post-ovulatory weeks). Such a study is of interest because: (1) A number of workers have reported on the embryonic and fetal development of the conducting system and the cardiac nerves and ganglia (Perman, 1924; Fukutake, 1925; Jones, 1932; Sanaliria, 1936; Walls, 1947; Robb, Kaylor & Turman, 1948; Licata, 1954; Navaratnam, 1965; Yamauchi, 1965; Smith, 1970; Cooper & O’Rahilly, 1971; O’Rahilly, 1971; Smith, 1971a, b; Anderson & Taylor, 1972; Asami, 1972; Dail & Palmer, 1973). (2) Electrocardiograms have been recorded from human fetuses (Heard, Burkley & Schaeffer, 1936), and an electrocardiogram with the classical P, QRS, and T configuration has been obtained from a 23 mm human embryo (Straus, Walker & Cohen, 1961). (3) Internodal, interatrial, and other specialized myocardial connexions have been described in the adult human heart and in several other species. Some of the more recent papers on the anatomy and histochemical features of these connexions are those by Robb & Petri (1961), James (1963), Emberson & Challice (1970), James (1970), James & Sherf (1971), Anderson (1972), Bojsen-Moller & Tranum-Jensen, 1972 and Tranum-Jensen & Bojsen-Moller (1973). Chuaqui (1972) has reviewed the literature on these connexions and their eponyms, and Janse & Anderson (1974) have published a critical analysis of the specialized functions that have been ascribed to them. Little information, however, is available about the presence of these connexions in the embryonic or fetal heart. (4) Evidence is available that catecholamine-containing cells, ganglion cells, and satellite cells can be identified and localized in fetal hearts in man and in other species (Jacobowitz, 1967; Friedman et al. 1968; Dail & Palmer, 1973).

• This work was supported in part by research programme project grant No. HD-08658, Institute of

Child Health and Human Development, National Institutes of Health (U.S.A.).

The aim of this report is to present information on the origin and distribution of the cardiac nerves, and the arrangements of the conducting system in the human heart at the end of the embryonic period proper.

MATERIAL AND METHODS

The stage 23 embryo of the present paper belongs to the Carnegie Collection and had a croyn-rump length of 31 mm. It was sectioned serially at 12 #m in a transverse plane and stained by the Mallory-azan technique. Although this technique is not specific for nerve fibres, it nevertheless demonstrated nerves with sufficient clarity for small filaments to be recognized easily.

The cardiac nerves and the various parts of the conducting system were meticulously followed in serial sections and their positions entered in drawings based on projections of individual sections. These drawings were the basis for Figure 2. In addition, reconstructions and diagrammatic representations were made of various features of the heart and conducting system (Figs. 1, 6, 13, 14).

OBSERVATIONS

The shape of the heart and the arrangement of the great vessels are shown in Figure 1.

Nerve supply Fig. 2)

The origin and distribution of nerves to the heart are shown schematically in Figure 2, which also shows the major differences between the right and left sides, as described below.

Right side. The nerve supply on the right side arose from several cervical vagal and sympathetic filaments that were interconnected and ganglionated in such a manner as to preclude any maintenance of separate vagal and sympathetic identity. The nerves may therefore be regarded as vagosympathetic (and afferent). The term “ganglion”, as used in the present paper, refers to collections of cells that comprise neuroblasts (immature neurons), chromaffin cells, satellite cells, and fibroblasts. Figures 3 and 4 illustrate a ganglion in which some of the cells have larger, paler nuclei. These are undoubtedly immature neurons, which may be sympathetic, parasympathetic, or sensory. The other cells, with smaller, more intensely staining nuclei, are probably chromaffin cells, the presence of which, early in prenatal development, has been demonstrated by fluorescence histochemistry (Jacobowitz, 1967; Friedman ef al. 1968; Dail & Palmer, 1973; Pappa, 1974; Kanerva & Hervonen, 1975).

Two filaments that arose at the level of origin of the recurrent laryngeal nerve formed a branch that crossed the median plane and entered the aorticopulmonary ganglion, from which it had a potentially widespread distribution to arterial and venous structures (ductus arteriosus, pulmonary trunk, ascending aorta, coronary arteries, pulmonary veins). This ganglion, which Licata (1954) termed the juxtaductal body, is shown in Figure 5 as well as in Figure 2. The filaments that arose from the right vagus nerve below the level of origin of the recurrent laryngeal nerve formed chiefly the right sinal nerve (nerve of the sinus venarum cavarum in the adult). This nerve first gave filaments that entered the sinu-atrial node (Figs. 10, 11) and then descended in the posterior part of the interatrial septum (Fig. 12). Here, small nerves and ganglion cells were present, the latter forming small clusters that could be traced across the median plane to similar clusters on the left side which were clearly related to the pulmonary veins. Ganglion cells and small nerve filaments were present in the interatrial septum as far distally as the coronary sinus, where they could be traced towards the posterior aspect of the atrioventricular node (Figs. 15, 17). None, however, appeared to enter the node.

Fig. 1. Graphic reconstruction of stage 23 heart in anterior and right lateral views. The pulmonary trunk is proceeding almost directly backwards and hence is seen ‘end on’ in the anterior view. The letters a, b, c, and d indicate the levels of the sections shown in (a) Figure 5, (b) Figures 24-26, (c) Figures 21 and 22, and (d) Figures 15-20. N.B. Abbreviations for all figures are given on p. 587.

No cardiac branches were seen to arise from the right sympathetic ganglia or trunk below the level of the ansa subclavia.

Left side. The nerve supply on the left side likewise arose from several cervical vagal and sympathetic ganglionated filaments, and several thoracic vagal filaments (Fig. 2). The cervical vagal and sympathetic filaments, including several from the recurrent laryngeal nerve, were directed chiefly towards the aorticopulmonary ganglion. Filaments from this ganglion were directed towards the pulmonary trunk, ductus arteriosus, ascending aorta, and pulmonary veins.

The two uppermost thoracic cardiac branches of the vagus descended to the left of the arch of the aorta and supplied chiefly the arterial end of the heart. Small ganglia were numerous in the area of distribution of these filaments. Other thoracic cardiac branches of the vagus arose at and immediately rostral to the level of origin of the recurrent laryngeal nerve. They united and, together with a filament from the aorticopulmonary ganglion, they formed the left sinal nerve (Fig. 2) as follows:

Fig. 2. Schematic representation of the origin and distribution of nerves to the heart, viewed from behind, and as if the nerves and ganglia occupied the same coronal plane. Superior and middle cervical ganglia and a cervicothoracic ganglion are present on the left side, whereas on the right there is a superior cervical ganglion and a long ganglionated mass that extends into the thorax. Cardiac nerves arising from the vagus nerves, and sympathetic ganglia and trunks, are shown as solid lines. The solid circles on or related to the cardiac nerves represent ganglia.

The filament from the aorticopulmonary ganglion descended in the left side of the venous mesocardium, where ganglion cells formed collections that were often continuous across the median plane with those related to the right sinal nerve. The descending filament divided. One branch entered the fold of the left vena cava and joined the filament derived from the thoracic cardiac branches of the left vagus nerve. This union formed the left sinal nerve (nerve of the coronary sinus in the adult); it accompanied the oblique vein of the left atrium, breaking up into several still smaller filaments as it did so. The vein and nerves could be traced almost as far as the coronary sinus. The other branch of the filament descending from the aorticopulmonary ganglion continued to descend in the left side of the venous Innervation of embryonic heart 575

mesocardium. It ended in groups of ganglion cells which were closely related to the pulmonary veins as they entered the left atrium.

No cardiac branches seemed to arise from the left sympathetic ganglia or trunk below the level of the ansa subclavia.

Conducting system

Sinu-atrial node. The sinu-atrial node formed a conspicuous mass that could be traced from rostral to caudal levels. It virtually encircled the superior vena cava, its anterior part being at a more rostral level than its posterior portion (Fig. 6). Beginning immediately below the entrance of the azygos vein, nodal tissue appeared in the anterior wall of the superior vena cava. From this level downwards, the node became much thicker and abutted against the posterior wall of the right atrium. At the same time, nodal tissue extended into the right and left walls of the superior vena cava, 50 that the node became crescentic in shape (Figs. 7, 8).

As seen with the Mallory-azan technique, the nodal cells had pale-staining nuclei and cytoplasm, and resembled those of the atrial myocardium. Collectively, however (in terms of general appearance and orientation), the nodal cells were distinct from the adjacent atrial myocardium (Figs. 7-9). This distinction was especially clear in front, where the node gave way abruptly to atrial myocardium. On the right side the node was likewise distinct from the crista terminalis, but caudally gave way more gradually to atrial myocardium. On the left side, the node gradually gave way caudally to atrial myocardium as the interatrial septum was formed (Fig. 12).

As the superior vena cava emptied into the right atrium, the nodal tissue gradually disappeared anteriorly. The anterior wall of the right atrium became thinner and then opened out as the septum spurium, which diverged caudally as the right and left venous valves. As the nodal tissue began to disappear anteriorly, it began to appear in the posterior wall of the superior vena cava (Fig. 12). This wall became thick and was continuous at the left with the posterior portion of the interatrial septum. The thickness of the posterior wall was maintained caudally into the intercaval fossa. Here, the nodal tissue gradually gave way to atrial myocardium, which could be traced into the posterior wall of the inferior vena cava, where it became thinner and disappeared.

In this embryo, at least three branches from the right sinal nerve were traced into the sinu-atrial node. However, no nodal artery was found.

Internodal connexions. Several bundles and sheets of myocardium which extended between the sinu-atrial and atrioventricular nodes were found in the present study. In terms of position, these corresponded in some respects to the internodal tracts described by James (1963, 1970) and by James & Sherf (1971). For example, the nodal tissue in the left and posterior walls of the superior vena cava merged caudally with the myocardium of the interatrial septum, which in turn merged with the posterior aspect of the atrioventricular node. The arrangement of the septum and the venous valves is shown in Figure 13. The septum secundum (S.2) diverges in front into the walls of the atria (Fig. 24). The portion that extends to the left continues caudally and merges with a myocardial mass from which the septum primum (S.1) extends posteriorly. The portion of S.2 that extends to the right forms a pro minent bundle (Fig. 23). This bundle could be traced caudally into a mass of myocardium which formed the expanded frontportion of S.1, above and behind the atrioventricular node. The same bundle could be traced to the right, away from S.2, above the tricuspid orifice. Although it was an integral part of the anterior wall of the right atrium, the bundle could nevertheless be traced below the tricuspid orifice into the right portion of the atrioventricular node. Hence, the bundle formed a ring around the atrial aspect of the tricuspid orifice.

Fig. 6. Diagram of the position of the sinu-atrial node in relation to the superior vena cava.

Viewed from the left side. The ring bundle is that which encircles the atrial aspect of the tricuspid orifice (see Fig. 23).

At slightly more caudal levels, the myocardial mass that formed the expanded front end of S.1 was continuous with a myocardial bundle that was less prominent than that on the right side, and which encircled the mitral orifice. Below, it merged With the left side of the most caudal portion of the atrioventricular node. Thus, the sinu-atrial node was connected with the atrioventricular node by way of the myocardium of the interatrial septum, and also by way of myocardial bundles that encircled the atrial aspects of the atrioventricular orifices, from the anterior portion of the interatrial septum above, to the atrioventricular node below.

Internodal connexions were also furnished by the septum spurium and the venous valves. The septum spurium, which was continuous above with the myocardium immediately in front of the sinu-atrial node, gave way below to the venous valves. The right venous valve then extended forwards to the right edge of the septum primum near the atrioventricular node and thereby formed the valve of the inferior vena cava. The left venous valve extended forwards to the myocardium around the opening of the coronary sinus immediately behind the atrioventricular node, and thereby formed a septum within the opening of the inferior vena cava.

Fig. 3. A ganglion on a cardiac nerve at the left side of the arch of the aorta. Magnification before reduction, x 110. N.B. This and all the other photomicrographs are oriented so that the posterior aspect is shown uppermost.

Fig. 4. The ganglion of Figure 3 shown at a higher magnification. Magnification before reduction, x 370.

Fig. 5. The caudal portion of the aorticopulmonary ganglion (arrow). Magnification before reduction, x 100.

Fig. 7. The sinu-atrial node (S.A.N) in the anterior wall of the superior vena cava. Note the sharp separation from the wall of the right atrium. Magnification before reduction, x 240.

Fig. 8. The sinu-atrial node somewhat caudal to the level of Figure 7. Note its increased thickness. Magnification before reduction, x 100.

Fig. 9. The sinu-atrial node and wall of right atrium of Figure 8. Magnification before reduction, x 370.

Fig. 10. At a level caudal to that of Figure 8. Nodal tissue is evident in the left wall of the superior vena cava. The right sinal nerve (R.S.N.) is evident. The arrows indicate filaments in the venous mesocardium. Magnification before reduction, x 150.

Fig. 11. The venous mesocardium of Figure 10, with the right sinal nerve (R.S.N.), ganglion cells (G.), and nerve filaments (arrows). Magnification before reduction, x 370.

Fig. 12. Nodal tissue completely encircles the superior vena cava, but is thinning anteriorly. The interatrial septum (S.2) contains ganglion cells and nerve filaments. Magnification before reduction, x 95.

Fig. 13. The septal region, based on several serial sections. The area enclosed by the rectangle in the left hand figure is shown at the right at a greater magnification.

Fig. 14. Diagrammatic representation of the components of the interatrial septum and the position of the atrioventricular node and bundle. Viewed from the left lateral aspect.

Whether the crista terminalis serves as an internodal connexion is doubtful. It is a bundle of atrial myocardium that merges at its left with the nodal tissue in the right and posterior walls of the superior vena cava. The junction is marked by venules (Fig. 12). The crista could be followed caudally to the level of the opening of the inferior vena cava, where it gradually disappeared.

Atrioventricular node (Figs. 14-20). The septum primum (S.1), which comprises the lower portion of the interatrial septum, consisted of myocardium that was expanded in front, as indicated above, and was continuous below with a mass of myocardium that was related to the opening of the coronary sinus. The atrioventricular node, a mass of pale-staining cells, lay mostly in front of the opening for the coronary sinus. It merged behind with the myocardium of the coronary sinus, and above with the myocardium of the interatrial septum. The right and left portions of the node were continuous with the myocardium that encircled the atrioventricular orifices. In front, the node was clearly distinct from the cardiac skeleton (Figs. 18, 19) and could readily be followed to the atrioventricular bundle (Fig. 20).

The atrioventricular node contained a small arteriole that arose more caudally from the right coronary artery near the posterior interventricular groove. Some of the small nerves and groups of ganglion cells, derived from or associated with the sinal nerves, and present in the posterior part of the interatrial septum, could be traced as far as the posterior and inferior edges of the node in the region of the opening of the coronary sinus.

The atrioventricular node, when followed upwards, shifted to the right, towards Innervation of embryonic heart 581

the interventricular septum. As it entered this septum, lying both in it and in the interatrial septum (S.1), the interventricular bundle could be said to begin (Fig. 20).

Atrioventricular bundle. As it ascended from the interventricular node, which it resembled in structure, the atrioventricular bundle comprised a round or oval mass, about 0-23 mm in its shortest diameter, which was partially enclosed in a thin, delicate sheath of parallel arranged cells with oval nuclei (Figs. 21-23). Merging with the septal myocardium in front, it was delimited behind by the collagenous framework of the developing cardiac skeleton. As it came to lie next to the cavity of the left ventricle, adjacent to a septal cusp, a group of cells extended to the left, immediately deep to the endocardium. This was the left limb, which could be traced for only a short distance. No cells that could be characterized as Purkinje fibres were found. Whether there were two such streams of cells comprising the left limb was difficult to determine. At any rate, thereafter the bundle became rounded to form the right limb (Fig. 24), which was about 0-1 mm in its shortest diameter and which, like the bundle itself, was somewhat demarcated by a few parallel arranged cells (Figs. 25, 26). It then came to lie immediately in front of the root of the aorta. Followed upwards and forwards, the limb gradually became smaller and smaller, and faded away into the interventricular septal myocardium. During its course, at least two streams of cells with dark-staining nuclei were noted proceeding from the region of the limb towards the septomarginal trabecula and the subendocardial region of the septum (right ventricular side). These probably comprised the branches of the right limb.

DISCUSSION

The importance of using staged embryos in developmental investigations has been emphasized in several recent general studies of the heart (Sissman, 1970; Goor, Edwards & Lillehei, 1970; O’Rahilly, 1971; Asami, 1972), as well as in an electron microscopic study of the development of nodal tissues (Yamauchi, 1965). The main components of the conducting system appear in recognizable form from 44 to 6

Fig. 15. The caudal portion of the atrioventricular node, which contains a nodal artery (N.4.). At a slightly more caudal level the right portion of the node is continuous with the bundle at its right (arrow). This bundle in turn can be traced rostrally around the tricuspid orifice (see Fig. 23). The uppermost arrow indicates a region between the coronary sinus (C.S.) and inferior vena cava (/.V.C.) which contains ganglion cells and a few nerve filaments. Magnification before reduction, x 60.

Fig. 16. The most caudal portion of the atrioventricular node, with nodal artery. Magnification before reduction, x 370.

Fig. 17. The atrioventricular node and interventricular septum at a more rostral level. Note the thick myocardium related to the coronary sinus (C.S.). The arrow indicates a region that contains ganglion cells and a few nerve filaments. Magnification before reduction, x 60.

Fig. 18. The atrioventricular node of Figure 17. Note sharp separation from interventricular septum. Magnification before reduction, x 150.

Fig. 19. The atrioventricular node of Figure 17 at a higher magnification. Magnification before reduction, x 370.

Fig. 20. At the level of the opening of the coronary sinus the atrioventricular node is becoming the atrioventricular bundle. The arrow indicates collections of ganglion cells. Magnification before reduction, x 150.

post-ovulatory weeks (about 6 to 12 mm C-R.), as follows: sinu-atrial node, stage 14; atrioventricular bundle, stage 15; atrioventricular node, stage 16; nerve fibres in the heart, probably stage 17. At 6 post-ovulatory weeks (about 3 weeks after the initiation of the heart beat), no varicose adrenergic fibres are present in the extrinsic nerves in or near the heart (Dail & Palmer, 1973).

At stage 23, the end of the embryonic period proper, and the subject of the present investigation, it would appear that the sinu-atrial node is supplied by vagosympathetic filaments that arise on the right side, whereas the arterial end of the heart is predominantly supplied by vagosympathetic filaments that arise on the left side (with some supply from the right side). The nerve fibres that are present in the interatrial septum and which reach the vicinity of the atrioventricular node arise chiefly from the right side. The pulmonary veins are supplied bilaterally, and ganglion cells are especially numerous in their vicinity. Our findings are comparable to those of Licata (1954) and of Navaratnam (1965).

The course of the vagosympathetic nerves and the pattern of the cardiac plexus at the end of the embryonic period proper do not necessarily indicate the adult topographical situation, and Dail & Palmer (1973) found that the adult pattern is not present until the fetus has attained about 70 mm in crown-rump length. Neuronal maturation, migration and differentiation continue, ganglia may regress, some filaments may remain microscopically small, and levels of origin may shift as nerves become bound in parent trunks for greater (or lesser) distances. Nevertheless, at stage 23, the nerve supply can be considered as forming, together with the conducting system, a functioning mechanism as indicated not only by an established circulation but also by the presence of characteristic electrical activity of the usual P-QRS-T complex (Straus ef al. 1961).

The absence of thoracic cardiac sympathetic branches is puzzling. (Licata (1954) commented that the thoracic sympathetic cardiac nerves were ‘vague”.) In the present study, small filaments were observed proceeding towards the aorta, oesophagus, and anterior longitudinal ligament (splanchnic nerves were also identified), but none could be traced to the heart. If this should be confirmed, it would mean that the thoracic sympathetic supply develops after the embryonic period proper. Moreover, the following findings need to be considered. Dail & Palmer (1973) found no varicose adrenergic fibres in embryonic human hearts. Kanerva &

Fig. 21. The septum primum (S.1) and the atrioventricular bundle (4.V.B.). Magnification before reduction, x 60.

Fig. 22. The atrioventricular bundle. Magnification before reduction, x 370.

Fig. 23. The atrioventricular bundle, immediately behind which is a portion of the developing cardiac skeleton. Immediately behind this, and indicated by an arrow, is a myocardial bundle proceeding rostrally from S.1 to S.2 and also to the bundle at the right of the tricuspid orifice (arrow). This bundle can be traced caudally to the right side of the atrioventricular node (see Fig. 15). Magnification before reduction, x 60.

Fig. 24. The right limb of the atrioventricular bundle. Theinteratrial septum is formed here by the septum secundum (S.2). Magnification before reduction, x 60.

Fig. 25. The right limb of Figure 24. Magnification before reduction, x 150.

Fig. 26. The right limb of Figure 24 at a higher magnification. Magnification before reduction, x 370.

Hervonen (1975) reported that the first cells in human sympathetic ganglia to show formaldehyde-induced fluorescence were small, intensely fluorescent cells (chromaffin cells) appearing at 7-8 weeks. In experimental animals catecholamine fluorescence, if present at all, is related to chromaffin cells, and not nerve fibres, in the early development of the heart (Jacobowitz, 1967; Friedman et al. 1968; Pappa, 1974). Hence, it must be considered possible that sympathetic fibres arising from cervical levels do not reach the heart during the embryonic period. If so, the term ‘vagosympathetic’ used in describing the cardiac nerve supply is a misnomer.

It is evident from a review of the literature that, in the usual histological studies, the distinctness of nodal tissue depends upon the staining method used. In the present study, the sinu-atrial and atrioventricular nodes were readily identified. We did not attempt to determine whether the atrioventricular node had a trilaminar arrangement, such as has been described by Anderson & Taylor (1972). What is surprising is that Licata (1954) found no atrioventricular node in the 25 mm (stage 22) and 31:5 mm (stage 23) embryos that he studied, and Anderson & Taylor (1972) found no evidence of an atrioventricular node in human embryos and fetuses until 46 mm, although they did report that, in a human embryo of 28 mm, a multicellular knot with cholinesterase-positive cells, together with nerve fibres, was present at the opening of the coronary sinus. We have no explanation to offer for this difference between our findings and theirs. Jones (1932) and Walls (1947) reported on the presence of the conducting system, including the atrioventricular node, in human embryos. Moreover, Yamauchi (1965) described the ultrastructural features of embryonic nodal tissue, beginning at stage 14 for the sinu-atrial node, and at stage 16 for the atrioventricular node.

The existence of internodal and interatrial connexions continues to be affirmed and denied (Davies, 1942; Anderson & Latham, 1971; Viragh & Challice, 1973; Janse & Anderson, 1974; Truex, 1974). There seems little doubt that internodal connexions do exist, and in the present study bundles and sheets of atrial myocardium could be traced from the sinu-atrial to the atrioventricular node. What is at issue, and what is beyond the province of the present paper, is the physiological and clinical significance of these connexions. Their potential importance in cardiac electrophysiology has been mentioned by a number of clinicians (Spurrell, Krikler & Sowton, 1973; Krikler, 1974), but the view that the connexions are physiologically specialized has been sharply challenged by Janse & Anderson (1974). Our own findings indicate simply that, at the end of the embryonic period, the myocardium that comprises the interatrial septum and the venous valves can be traced from the sinu-atrial to the atrioventricular node. The myocardium that encircles the atrioventricular orifices, especially on the right, resembles the encircling ring studied by Shaner (1929) in the calf heart (a ring that disappears before birth). The continuation of the right ring into the anterior wall of the right atrium, as noted in the present study, may correspond to what has been termed the interatrial bundle of the adult heart.

SUMMARY

The nerve supply and conducting system were studied in a stage 23 human embryo of exceptional histological quality.

The nerves on the right side arose from cervical sympathetic and from cervical and thoracic vagal filaments. Out of their interconnexions vagosympathetic nerves emerged, which (1) sent a branch in front of the trachea to the aorticopulmonary ganglion, thereby supplying arterial and venous structures, and (2) formed the right sinal nerve, which supplied the sinu-atrial node, and gave filaments to the interatrial septum which could be traced to the atrioventricular node and pulmonary veins.

The nerves on the left side arose similarly from cervical sympathetic and from cervical and thoracic vagal filaments. These formed several descending, ganglionated, vagosympathetic filaments that descended to the right of the arch of the aorta and entered the aorticopulmonary ganglion. Filaments leaving the ganglion supplied the pulmonary trunk, ascending aorta, interatrial septum, pulmonary veins, and, as the left sinal nerve, the fold of the left vena cava. The thoracic vagal filaments descended to the left of the arch of the aorta and supplied chiefly the arterial end of the heart.

No thoracic sympathetic cardiac filaments were found.

The sinu-atrial node began as a crescentic mass in front of the lower part of the superior vena cava. It gradually extended on each side of the superior vena cava and came to form its posterior wall at a more caudal level. The atrial myocardium that formed the septum spurium, venous valves, and interatrial septum could be traced from the sinu-atrial to the atrioventricular node. Myocardium also encircled the atrial aspects of the atrioventricular orifices, and could be traced caudally to the atrioventricular node.

The atrioventricular node was a conspicuous mass in the anterior and lower part of the interatrial septum, from which a clearly defined bundle left to enter the interventricular septum. Right and left limbs were observed, the former being a rounded bundle that passed immediately in front of the root of the aorta.

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ABBREVIATIONS USED IN FIGURES

À. Aorta L.S. Left subclavian artery A.V. Aortic valve L.V. Left ventricle

A.V.B. Atrioventricular bundle L.V.V. Left venous valve

AVN Atrioventricular node N.A Nodal artery

B.C.TI Brachiocephalic trunk P.T. Pulmonary trunk

CS Coronary sinus P.V. Pulmonary valve

CT. Crista terminalis R.A. Right atrium

D.A Ductus arteriosus R.C.C. Right common carotid artery F2 Foramen secundum R.L.N. Recurrent laryngeal nerve F.0. Foramen ovale RS. Right subclavian artery G. Ganglion or ganglion cells R.S.N. Right sinal nerve

LvV.C. Inferior vena cava R.V. Right ventricle

LV.S. Interventricular septum R.V.V. Right venous valve

L.10 Left vagus nerve S.1 Septum primum

L.A. Left atrium S.2 Septum secundum

L.C.C. Left common carotid artery S.-A.N. Sinu-atrial node

L.R.L.N. Left recurrent laryngeal nerve S.V.C. Superior vena cava