Double outlet right ventricle in a calf

Double outlet right ventricle is a conotruncal malformation where both great arteries (aorta and pulmonary trunk) arise from the right ventricle. A 2-month-old Holstein calf was slaughtered due to severe respiratory distress. At necropsy, the heart was enlarged, globose, and had ventricular and atrial septal defects. The only outlet for the left ventricle was a large ventricular septal defect located at 6 cm distance from the heart apex and involved atrial septum too. The right ventricle was enlarged and markedly thickened with a left to right free wall ratio of 2.5:2 and prominent papillary muscles. The aorta arose from the right ventricular infundibulum adjacent to pulmonary trunk. Two valvular hematomas were observed on the edge of the right atrioventricular valve. The lungs were rubbery with ecchymotic and petechial hemorrhages and did not collapse after removing from thoracic cavity. Enhanced lobular pattern was evident on both the capsular and cut surfaces of the liver. Histopathological examination of the lungs revealed thickening of alveolar septa, hemorrhages, and infiltration of hemosiderophages within alveoli. Periportal hepatocellular fatty changes, substitution of centrilobular and midzonal hepatocytes by red blood cells, and dilation of midzonal and periportal sinusoids were seen in the liver. To our knowledge, this particular combination of cardiac defects has not been previously described in domestic animals and the pathological lesions observed in the calf may be resulted as a sequel to left–right blood shunting and heart failure.     

Development of the inlet portion of the right ventricle in the embryonic rat heart: The basis for tricuspid valve development

Background: Before septation the entire atrioventricular canal is connected with the ventricular inlet segment (primitive left ventricle), wheres the mature heart exhibits an exclusive connection of the right atrium to the right ventricule. The process which is responsible for this change is controversial. Methods: Graphic reconstructions of serially sectioned embryonic rat hearts as well as scanning electron micrographs of similar specimens were made. Results: The first indication of a right atrioventricular connection was seen as a groove in the atrioventricular junctional myocardium to the right of the inferior endocardial cushion. This groove expanded to form the right ventricular inlet portion. The right, inferior, and superior walls of this newly formed cavity were formed from junctional myocardium, which demarcated it from the trabeculated right ventricular portion in all developmental stages. The left wall equally developed from this junctional myocardium and formed the ventricular inlet septum. The junctional myocardium between right ventricular inlet and trabeculated portions was seen to develop into the tricuspid valve and its tension apparatus. Conclusions: The preseptation embryonic heart has no inlet portion to the right ventricle. This new cavity is created by remodelling of atrioventricular junctional myocardium. This myocardium also provides the material contribution to the tricuspid valve and its tension apparatus. Malformations of the right ventricular inlet portion and of the tricuspid valve are indissolubly linked. © 1994 Wiley-Liss, Inc.     

Gross heart anatomy of <Emphasis Type="Italic">Arctocephalus australis</Emphasis> (Zimmerman, 1783)

Little research has been carried out on the gross visceral anatomy of the Otariidae, and the anatomical information for the southern fur seals, Arctocephalus spp., is scant. The aim of the present study was to describe the external and internal conformation, and the sanguineous irrigation of the heart of Arctocephalus australis. Twelve hearts of Arctocephalus australis were studied by simple dissection. In the right ventricle the trabeculae carneae were well developed and there were three or more papillary muscles. In the left ventricle there were two papillary muscles, subatrialis and subauricularis, attached to the parietal wall. There was also a great development of trabeculae carneae which occupied almost all of the ventricle, from the left atrioventricular valve up to the proximities of the expulsion route. A large quantity of muscular strands were found extending themselves between the trabeculae carneae, becoming more dense and forming a network when near the apex. The distribution of the branches of the coronary arteries was highly variable and no heart was similar to another one in this sense. In the majority of the hearts the subsinosal interventricular branch proceeded from the right coronary artery. It is concluded that there were many differences between the heart of the Arctocephalus australis and the heart of the domestic dog, contrary to what has been suggested for other genera of Otariidae.     

Inner ventricular structures and valves of the heart in white rhinoceros (Ceratotherium simum)

In this study, we describe the internal structures of both ventricles and the valvular apparatus of the heart of the white rhino. In the right of the heart, three papillary muscles were found in septal and marginal walls and m. papillaris magnus was the biggest. There was only one m. papillaris parvus in the right ventricle. The right atrioventricular valve was tricuspid, and the parietal cusp was longest. In the left of the heart, two papillary muscles were found on the septal wall and the subauricular was the biggest. The left atrioventricular valve was bicuspid and the parietal cusp was longest. There were no nodules in the valves of the pulmonary trunk and aorta, and the semilunar valves had many fibrous folds and transparent parts. Within the cardiac skeleton there was a cartilago cordis which occupied a small part of the right fibrous trigone. While the right ventricle included only one septomarginal trabecula, there were many trabeculae in the left ventricle. In both ventricles, the endocardium was thin and the subendocardial network was visible, also their continuation with the septomarginal trabeculae. We also found many trabeculae carneae in the dorsal part of the ventricles.     

Histological organization of the right and left atrioventricular valves of the chicken heart and their relationship to the atrioventricular purkinje ring and the middle bundle branch

In the avian heart the right and left atrioventricular (AV) valves not only exhibit their own special anatomical characteristics, but they also are in close proximity to the conduction system. The right AV valve is a single, spiral plane of myocardium, in remarkable contrast to the fibrous structure characteristic of the mammalian tricuspid valve. A ring of Purkinje tissue encircles the avian right AV orifice and connects to the muscular valve. The chicken has no crista supraventricularis, its right AV valve serving that function as well as opening and closing the right AV orifice. The left AV valve consists of three leaflets instead of the two typical of mammalian hearts. Its anterior and posterior leaflets are small; its large aortic (medial) leaflet merges with the bases of both the left and noncoronary cusps of the aortic valve by fibrous tissue, resembling that of the mammalian heart. However, unlike in mammals, there is a slim cylinder of continuous myocardium coursing parallel to this fibrous junction. This unusual arc of myocardium in the chicken serves to complete an entire subaortic ring of myocardium and is thus potentially capable of constricting the outflow tract of the chicken's left ventricle. The middle bundle branch connects with both the muscle arch and the AV Purkinje ring. Thus the myocardium in or near both AV valves (and the left ventricular outflow tract) in the chicken heart is so arranged that it may receive direct early activation from the conduction system. ©1993 Wiley-Liss, Inc.     

The uptake of radioactive phosphorus (P32) by various structures of the heart

Summary Experiments were performed on rabbits. There was a definite topography of radiophosphorus (P³²) inclusion into different portions and structures of the heart.The intensity of this inclusion decreased in the following sequence: left ventricle (maximal intensity), right ventricle, left auricle, right auricle. It was also demonstrated that in experimental conditions the radioactivity of the left ventricle is much greater (by 3–4 times) than that of the other portions of the heart and blood. The centers or nerve ganglions governing automatic control are characterized by a minimal intensity of the P³² inclusion. The inclusion of radiophosphorus is more intense in the elements of the bundle of His than in the sino-auricular and atrioventricular nodes.Presented by Active Member AMN SSSR B. N. Man'kovskii     

The structure of the atrioventricular conducting system in the avian heart

The atrioventricular conduction system in three avian species has been studied by light and electron microscopy. A morphologically definable atrioventricular node was not found in any of these. The atrioventricular bundle is a well-defined structure, the proximal portion of which is in direct continuity with the atrioventricular ring, located in the arterial sheet of the muscular valve of the right atrioventricular opening. In the zone of transition between atrioventricular ring and bundle the compactness of the bundle is loosened, but the fibers do not establish continuity with the atrial fibers. The ring consists of Purkinje-like fibers, 10–15 μm in diameter, and (peripherally) small 3–5-μm-diameter junctional fibers which are in continuity with the common atrial fibers. In the muscular atrioventricular valve the fibers of the ring are insulated from the ventricular myocardium by a connective tissue sheet of the annulus fibrosus. It is suggested that in the avian heart the atrioventricular ring may fulfill a role similar to that of the atrioventricular node of mammals.     

Histological examination of the potential arrhythmic substrates in the setting of Ebstein’s malformation

Cardiac arrhythmias, notably Wolff-Parkinson-White syndrome, are known to represent a major issue in patients with Ebstein’s malformation of the tricuspid valve. Abnormal conducting circuits, however, can also be produced by pathways extending either from the atrioventricular node or the ventricular components of the atrioventricular conduction axis, direct to the crest of the muscular ventricular septum. We hoped to provide further information on the potential presence of such pathways by investigations of six autopsied examples of Ebstein’s malformation. All were studied by histological sectioning on the full extent of the atrioventricular conduction axis, with limited sectioning of the right atrioventricular junction supporting the inferior and antero-superior leaflets of the deformed tricuspid valve. We used the criteria established by Aschoff (Verhandlungen der Deutschen Gesellschaft für Pathologie, 14, 1910, 3) and Mönckeberg (Verhandlungen der Deutschen Gesellschaft für Pathologie, 14, 1910, 64) over a century ago to define abnormal connections across the atrioventricular junctions, as these definitions retain their validity for the identification of gross myocardial connections across the insulating tissues of the atrioventricular junctions. In one specimen, we found two discrete accessory myocardial connections across the parietal right atrioventricular junction. In all of the hearts, we found so-called nodoventricular connections, and in one heart we also observed a well-formed connection originating from the penetrating atrioventricular bundle. In addition to accessory myocardial connections across the parietal right atrioventricular junction, therefore, our histological findings demonstrate a potential role for direct connections between the atrioventricular conduction axis and the ventricular myocardium in the setting of Ebstein’s malformation.     

Study of the development of the atrioventricular conduction system as a consequence of observing an extra atrioventricular node in the normal heart of a human fetus

We have observed an extra atrioventricular node in the normal heart of a human fetus. It is located in the septal wall of the right atrium, subendocardially, and just where Todaro's tendon leaves this wall to go toward the inferior vena cava valve. In its trajectory, this tendon gives way to a remarkable prominence in the cavity of the right atrium: the sinus band. In order to explain the embryogenesis of this extra atrioventricular node, we have studied the normal development of the atrioventricular specific system and have concluded that the atrioventricular node is formed from a growth and displacement toward the atrium of the primitive atrioventricular specific material, which originates from the myocardium of the posterior wall of the atrioventricular canal. Likewise, during its development, the atrioventricular node keeps in close proximity with the Todaro's tendon. In our view, this accounts for the embryogenesis of the extra atrioventricular node, since a fragment of the atrioventricular node can remain cranial to Todaro's tendon and be displaced by it in a craniodorsal direction. This fragment would then lead to the formation of an extra atrioventricular node like the one present in the heart of the fetus we have examined.     

Right ventricular false tendons, a cadaveric approach

Left ventricular false tendons (LFTs) have been extensively described and recognized by gross anatomic studies. However, there is very little information available regarding right ventricular false tendons (RFTs). The aim of our study, therefore, was to explore and delineate the morphology, topography and morphometry of the RFTs, and provide a comprehensive picture of their anatomy across a broad range of specimens. We identified 35/100 heart specimens containing right ventricular RFTs and classified them into five types. In Type I (21, 47.7%) the RFTs, was located between the ventricular septum and the anterior papillary muscle; in Type II (11, 22.9%) between ventricular septum and the posterior papillary muscle; in Type III (7, 14.5%) between the anterior leaflet of the tricuspid valve and the right ventricular free wall; in Type IV (5, 10.4%) between the posterior papillary muscle and the ventricular free wall; and lastly, in Type V (4, 8.3%) between the anterior papillary muscle and ventricular free wall. The mean length of the RFTs was 18 ± 7 mm with a mean diameter of 1.4 ± 05 mm. Histologic examination with Masson trichrome and PAS revealed that 20 (41.6%) of the 48 RFTs carried conduction tissue fibers. The presence of conduction tissue fibers within the RFTs was limited to Types I, III, and IV. In Types II and V the RFTs resembled fibrous structures in contrast with Type I, II and IV, which were composed more of muscular fibers, including conduction tissue fibers. RFTs containing conduction tissue fibers were identified, which may implicate them in the appearance of arrhythmias.     

The cardiac malformations: Souble inlet left ventricle and corrected transposition explained as deviations in the normal development of the interventricular septum

To examine the hypothesis that malpositions of cardiac ventricles could be explained by altered development of the interventricular septum, we studied hearts from The Johns Hopkins Hospital autopsy files with double inlet left ventricle (16 cases) or corrected transposition (nine cases). In double inlet left ventricle both atrioventricular valves connect normally developed and positioned right and left atria to a posterior morphologic left ventricle. In hearts with corrected transposition the atria are normally positioned and the morphologic right ventricle is on the left and is continuous with the anteriorly positioned aorta. The morphologic left ventricle is on the right, connected to the posteriorly positioned pulmonary trunk. Normal ventricular septation may be understood as arising from the mechanics of a spiral fold in the primary heart tube produced by the left interventricular sulcus. The ventral limb of the spiral induces the muscular interventricular septum while the dorsal limb becomes a component of the crista supraventricularis.We propose that double inlet left ventricle and corrected transposition are the result of minor deviations in the position of the interventricular sulcus on the primary heart tube. Double inlet left ventricle may develop from the formation of a closed unspiraled ring around the interventricular canal. Corrected transposition may result from a left interventricular sulcus whose ventral limb gives rise to a left sided crista supraventricularis, which determines in part the right ventricular morphology of the left sided ventricle. The dorsal limb spirals toward the atrioventricular canal, gives rise to a malpositioned interventricular septum, and displaces the embryonic trabeculated right ventricle to the left. The concept presented accounts for the morphologic findings characteristic of double inlet left ventricle and corrected transposition.     

Cardiac function in open-chest dogs after left to right ventricular shunting and right coronary artery occlusion

Patients with acquired ventricular septal defect (VSD) after myocardial infarction have a particularly bad prognosis if right ventricular function is severely impaired. The significance of an ischaemic right ventricular free wall on cardiac function during interventricular shunting was examined in open-chest dogs. An external interventricular shunt could be opened and closed at will, and by occlusion of the right coronary artery (RCA), a part of the right ventricular free wall was rendered ischaemic. Aortic flow decreased by 8 +/- 2% when the shunt was opened in the presence of a normal right ventricle, and by 16 +/- 2% (difference: P less than 0.05) in the presence of right ventricular ischaemia. Aortic flow fell by 19 +/- 3% when the RCA was occluded. Right ventricular dyskinesia was demonstrated after occlusion of RCA, by recording segment lengths in the right ventricular free wall. The dyskinesia was aggravated when the shunt was opened. The left ventricle exerted a 'negative' work on the ischaemic right ventricular free wall. Retention of blood in the right ventricle, with a subsequent decline in left ventricular filling and an almost unchanged interventricular shunting of blood, explain why aortic flow fell more when the shunt was opened in the presence of right ventricular ischaemia.     

Atresia of the pulmonary artery with intact intraventricular septum

Cardiological studies of 16 patients with atresia of the pulmonary artery (APA) and intact interventricular septum (IIS) provided evidence for the following inferences: 1) most hearts in this pathology are characterized by right ventricular (RV) hypoplasia which determines structural defects of the ventricle dependent in their severity on restriction capacity of the right atrioventricular valve and time from the onset of reduced RV blood flow 2) common anatomical features of the defect are complete RV obstruction due to valvular atresia, IIS and normal shape of the conus, RV myocardial hypertrophy 3) survival of the patients conditioned by interatrial communication and open arterial canal. According to the authors, there are three types of APA with IIS. Type I--normal right ventricle with distinct components: sinuous, conical, trabecular. Type II--hypoplastic right ventricle with blocked function of the trabecular component. Type III--right ventricle affected by advanced hypoplasia and with operable sinuous part only.     

Comparative morphology of the trabecula septomarginalis in terrestrial mammals]

The authors studied macroscopic morphology and histologic structure of the moderator band - trabecula septomarginalis - in 100 earthly mammals. A real trabecula septomarginalis was never found in carnivora (dog and fox) because the "anterior" papillary muscle of the right ventricle is located on the lower part of the septum ventriculorum or at times bridge-shaped above the anteiror interventricular groove and attached on both septum and anterior wall of the right ventricle. On the opposite, in ungulates, a well-known band runs across the right ventricular chamber from the septum close to the musculus papillaris coni arteriosi up to the anterior wall close to the anterior papillary muscle. In fact the attachments lie more or less approximate to the pillars; in suidae, the trabecula usually ends on a vertical trabecula carnea just under Luschka's muscle. Three types of trabecula septomarginalis were encountered as previously described by Bortolami in ox: - Mostly (66%), the trabecula septomarginalis is a short and thick fleshy column. The ratio Length mm Thickness mm amounts to about 4 to 7 and can be regarded as reasonably constant within each variety. Moreover the quotients Length mm Height of septum MM. and Thickness of the trabecula mm Highest thickness of the anterior wall mm (measured just beneath the tricuspid attachment) keep constant in each group; thus it may be concluded that size and shape of the trabecula depend on the volume of the heart and the age of the animal. - Rarely, the trabecula looks like a tendinous cord. Such a fibrous string is nearly constant in suidae, especially in wild boar, but seldom in bovidae and cervidae. In all cases, the trabecula septomarginalis supports the right branch of the atrioventricular bundle and a thin artery which originates from the left coronary artery and branches into the right coronary vessels within the anterior papillary muscle. Some venous capillaries were also observed but only in the muscular trabeculae; they are not constant and mouth into the right ventricular chamber or run towards the septal veins around the atrioventricular node. Therefore whatever is the size, the trabecula septomarginalis must be regarded as the shortest pathway from the septum to the anteiror wall of the right ventricle and is a mere band bearing the "right nodal pedicle".     

Left atrial ligation alters intracardiac flow patterns and the biomechanical landscape in the chick embryo

Background: Hypoplastic left heart syndrome (HLHS) is a major human congenital heart defect that results in single ventricle physiology and high mortality. Clinical data indicate that intracardiac blood flow patterns during cardiac morphogenesis are a significant etiology. We used the left atrial ligation (LAL) model in the chick embryo to test the hypothesis that LAL immediately alters intracardiac flow streams and the biomechanical environment, preceding morphologic and structural defects observed in HLHS. Results: Using fluorescent dye injections, we found that intracardiac flow patterns from the right common cardinal vein, right vitelline vein, and left vitelline vein were altered immediately following LAL. Furthermore, we quantified a significant ventral shift of the right common cardinal and right vitelline vein flow streams. We developed an in silico model of LAL, which revealed that wall shear stress was reduced at the left atrioventricular canal and left side of the common ventricle. Conclusions: Our results demonstrate that intracardiac flow patterns change immediately following LAL, supporting the role of hemodynamics in the progression of HLHS. Sites of reduced WSS revealed by computational modeling are commonly affected in HLHS, suggesting that changes in the biomechanical environment may lead to abnormal growth and remodeling of left heart structures. Developmental Dynamics 243:652–662, 2014. © 2013 Wiley Periodicals, Inc.     

Remodeling of chick embryonic ventricular myoarchitecture under experimentally changed loading conditions

Adult myocardium adapts to changing functional demands by hyper‐ or hypotrophy while the developing heart reacts by hyper‐ or hypoplasia. How embryonic myocardial architecture adjusts to experimentally altered loading is not known. We subjected the chick embryonic hearts to mechanically altered loading to study its influence upon ventricular myoarchitecture. Chick embryonic hearts were subjected to conotruncal banding (increased afterload model), or left atrial ligation or clipping, creating a combined model of increased preload in right ventricle and decreased preload in left ventricle. Modifications of myocardial architecture were studied by scanning electron microscopy and histology with morphometry. In the conotruncal banded group, there was a mild to moderate ventricular dilatation, thickening of the compact myocardium and trabeculae, and spiraling of trabecular course in the left ventricle. Right atrioventricular valve morphology was altered from normal muscular flap towards a bicuspid structure. Left atrial ligation or clipping resulted in hypoplasia of the left heart structures with compensatory overdevelopment on the right side. Hypoplastic left ventricle had decreased myocardial volume and showed accelerated trabecular compaction. Increased volume load in the right ventricle was compensated primarily by chamber dilatation with altered trabecular pattern, and by trabecular proliferation and thickening of the compact myocardium at the later stages. A ventricular septal defect was noted in all conotruncal banded, and 25% of left atrial ligated hearts. Increasing pressure load is a main stimulus for embryonic myocardial growth, while increased volume load is compensated primarily by dilatation. Adequate loading is important for normal cardiac morphogenesis and the development of typical myocardial patterns. Anat Rec 254:238–252, 1999. © 1999 Wiley‐Liss, Inc.     

Revisiting the anatomy of the right ventricle in the light of knowledge of its development

Controversies continue regarding several aspects of the anatomy of the morphologically right ventricle. There is disagreement as to whether the ventricle should be assessed in bipartite or tripartite fashion, and the number of leaflets to be found in the tricuspid valve. In particular, there is no agreement as to whether a muscular outlet septum is present in the normally constructed heart, nor how many septal components are to be found during normal development. Resolving these issues is of potential significance to those investigating and treating children with congenitally malformed hearts. With all these issues in mind, we have revisited our own experience in investigating the development and morphology of the normal right ventricle. To assess development, we have examined a large number of datasets, prepared by both standard and episcopic microscopy, from human and murine embryos. In terms of gross anatomy, we have compared dissections of normal autopsied hearts with virtual dissections of datasets prepared using computed tomography. Our developmental and postnatal studies, taken together, confirm that the ventricle is best assessed in tripartite fashion, with the three parts representing its inlet, apical trabecular, and outlet components. The ventricular septum, however, has only muscular and membranous components. The muscular part incorporates a small component derived from the muscularised fused proximal outflow cushions, but this part cannot be distinguished from the much larger part that is incorporated within the free-standing muscular infundibular sleeve. We confirm that the tricuspid valve itself has three components, which are located inferiorly, septally, and antero-superiorly.     

Dependence of Aortic Arch Morphogenesis on Intracardiac Blood Flow in the Left Atrial Ligated Chick Embryo

Partial left atrial ligation before cardiac septation redistributes intracardiac blood flow and produces left ventricular hypoplasia in the chick. We hypothesized that redistributed intracardiac blood flow adversely alters aortic arch development. We ligated the left atrial appendage with a 10‐0 nylon suture at stage 21 chick embryos, then reincubated up to stage 34. Sham embryos had a suture tied adjacent to the atrial wall, and normal controls were unoperated. We measured simultaneous atrioventricular (AV) and dorsal aortic (DAo) blood velocities from stage 24 embryos with an ultrasound pulsed‐Doppler flow meter; and the left and right third and fourth aortic arch blood flow with a laser‐Doppler flow meter. Ventricular and atrial cross‐sectional areas were measured from sequential video fields for planimetry. Intracardiac flow patterns were imaged on video by injecting India ink into the vitelline vein. In separate embryos, radiopaque microfil was injected into the cardiovascular system for μ‐CT scanning. We analyzed the morphologic characteristics of the heart at stage 34. Active AV and DAo stroke volume (mm³), right third and fourth aortic arch blood flow (mm³/s) were all decreased in ligated embryos (P < 0.05) when compared with normal and sham embryos. Ventricular end‐diastolic volume versus normal and sham embryos decreased by 45% and 46%, respectively (P < 0.05). India ink injection revealed altered right aortic arch flow patterns in the ligated embryos compared with normal embryos. μ‐CT imaging confirmed altered arch morphogenesis. Alterations in intracardiac blood flow disrupt both early cardiac morphogenesis and aortic arch selection. Anat Rec, 2009. © 2009 Wiley‐Liss, Inc.     

成人右房室瓣瓣叶分区与形态的解剖观测

目的为心脏右房室瓣病变的临床诊治提供解剖学资料。方法对171个成人心脏右房室瓣瓣叶分区进行形态学测量。结果右房室瓣瓣叶分为粗糙区、透明区、基底区。瓣膜形状有三角形、半椭圆形、半圆形、长方形和梯形。双前瓣瓣叶高度最高,内瓣瓣叶高度最低。前瓣透明区和基底区室面腱索最少,因而前瓣活动性最大。内瓣和后内瓣室面附着的腱索多而短小,使瓣叶活动性最小。结论心脏外科手术中应密切注意瓣叶的解剖学特点。     

Role of the left interventricular sulcus in formation of the interventricular septum and crista supraventricularis in normal human cardiogenesis

Serial sections of normal human embryos were studied and three-dimensional images reconstructed to determine the early development of the interventricular septum. The position of the interventricular septum is determined in stage 9 of normal development by the formation of the left interventricular sulcus. As a result of unknown properties of the cells of the myocardial layer, the left interventricular sulcus persists while the right disappears, producing the initial lateral asymmetry of the primary heart tube. By stage 14, the left interventricular sulcus forms a spiral which is continuous with the developing interventricular septum. The dorsal limb of the spiral passes to the right between the atrioventricular canal and the origin of the outflow tract, and is lost in the wall of the trabeculated right ventricle. It appears that this dorsal limb of the spiral is the precursor of part of the cirsta supraventricularis. The midportion of the sulcus, the bulboventricular groove, becomes the socalled fibrous continuity between the aortic and mitral valves. The ventral limb of the spiral passes caudally in the anterior interventricular groove and then dorsally and cranially toward the dorsal cushion of the atrioventricular canal. The ventral limb of the spiral is continuous with the crest of the muscular interventricular septum, which develops by apposition of tissue from the expanding right and left ventricles. From stage 14 to stage 19, the muscular interventricular septum, the atrioventricular endocardial cushions, and the ventricular end of the spiral ridges of the outflow tract appose and fuse. Subsequent formation of the membranous interventricular septum completes the physical separation of the right and left ventricles.