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The cardiac specialized conduction tissue was studied by serial sectioning in 13 cases of situs ambiguus. In four cases of right isomerism, we found paired sinus nodes in relation to a crista terminalis, and in each case a sling of conduction tissue between two atrioventricular nodes was present regardless of the ventricular morphology or cardiac position. In the cases with left isomerism, the sinus node was hypoplastic and abnormally located. We saw two types of atrioventricular conduction systems. In the three cases in which the morphologically right ventricle lay to the right of the morphologically left ventricle (presumed d-loop), a single atrioventricular bundle arose from a normally located atrioventricular node. In the five cases in which the morphologically right ventricle lay to the left of the morphologically left ventricle (presumed l-loop) and in the one case with a univentricular heart, paired atrioventricular nodes were present, linked or potentially linked by a sling of conduction tissue.

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suMMARY Discrete delicate fibromuscular structures crossing the cavity of the left ventricle were identified on morphological examination in 329 (48%) of 686 hearts from patients of all ages with congenital heart disease, acquired heart disease, or normal hearts. These structures were also present in 151 (95%) of 159 hearts from animals of six species. Cross sectional echocardiographic findings compatible with these structures were obtined in 39 (21-7%) of 179 children reviewed retrospectively and in three of 800 (0-4%) adults studied prospectively. These structures appear to be a normal anatomical finding.Isolated examples of filaments of tissue crossing the cavity of the left ventricle were first reported in 1893 by Turner. '-3 In 1906 Keith and Flack noted that they were almost constantly present in both bovine and human hearts and they regarded them as being part of the left bundle branch distribution of the Purkinje conduction fibres.4 Although these structures are hardly mentioned in the standard textbooks of anatomy and cardiac pathology,56 the development of echocardiography appears to have reawakened interest in them.7-1 ' We made a quantitative morphological assessment of the prevalence of these structures as this would seem to be a prerequisite for an appraisal of their importance. For reasons that we will discuss later we have called these structures left ventricular bands. Patients and methodsWe reviewed 686 hearts in the collection of this hospital specifically for the presence of left ventricular bands. Six hundred and thirty six were from children below the age of 15. Of these, 581 had had congenital cardiac malformations and 55 had had normal hearts. The 50 other hearts were from adults who had previously had surgery for acquired cardiac disease. We also examined 159 hearts from six species of animals Accepted for publication 31 July 1984 (none of which was killed for the purpose of the study). Ventricular bands were identified as discrete fibromuscular structures crossing the left ventricular cavity and having no attachment to the atrioventricular valve (Fig. 1). They extended from a papillary muscle to the septum, from a papillary muscle to the free wall, between the papillary muscles, or from the septum to the free wall. They were either single or multiple and sometimes branched. Small multiple strands at the junction of the setpum with the free walls were disregarded as were small apical networks.In addition to the morphological study, we reviewed cross sectional echocardiograms from 800 adults and 179 children. Adult echocardiograms had been recorded on a Varian V3400 phased array system with a 2*2 MHz transducer.