The concept of discrete layers and bundles of muscle as a basic structural arrangement in left ventricular myocardium was tested by measuring the helix angles at 1 m m intervals from endocardium to epicardium, using pig's heart in the contracted state. A fixed coordinate system was established which permitted measurement of corresponding sites in hearts of different dimensions.The helix angle was found to change from somewhat less than 90" endocardially to about -90" epicardially in a n almost linear clockwise sequence, like a Japanese fan opened up. Approximately the same pattern was observed in the interventricular septum and the anterior, left and posterior walls. Generally, there was no abrupt change between the helix angle of papillary fibers and that of adjacent wall myocardium. Where occasionally abrupt changes in fiber orientation were demonstrated, no intervening septum could be discerned. The deviant fibers seemed to co-exist as part of the same gross structure.The concept of a continuum more appropriately describes the basic structure of left ventricular myocardium. Lev and Simkins ('56) and Grant ('65) showed that there was no evidence of identifiable layers as defined by the presence of connective tissue septa. This study shows no evidence of identifiable layers as defined by (1) a n abrupt change in fiber direction demarking the boundary of a layer and (2) a parallel fiber direction between such boundaries.For the past five centuries, prominent anatomists have been asserting that the ventricles are made up of discrete, overlapping muscle bands, each band being consistently identifiable. The history of this account of heart morphology beginning with Vesalius in 1514 has been reviewed by Robb and Robb ('42). In this view, myocardial structure is generally visualized like the layers of an onion, each layer of muscle fiber separated by wellmarked septa and having its own characteristic fiber direction. The standard unwinding technique for unraveling muscle bundles lends support to this idea.Recently Grant ('65) challenged the use of the unwinding technique. In this procedure, as employed by such anatomists as Mall ('ll), Shaner ('23) or Thomas ('57), a decollagenized muscle bundle is split off from the main muscle mass along a "natural" cleavage plane. However, myocardium is a syncytium with branch points. If there were no septa, one fiber would be ruptured when the cleavage plane reaches a point of fiber branching. Lev and Simkins ('56), reemphasized by Grant ('65), found that in unwinding the outer myo-ANAT. REC.. 155: 503-512.cardial fiber bundles, the choice of fiber to be ruptured depends upon the way the bundle is split off. Different populations of unwound fibers exposed different helix angles. The so-called "discrete" muscle bands do not have discrete "natural" cleavage planes between bundles. Rather, he observed, the helix angles in myocardial layers are continuous with each other and have a gradual transition from epicardium to endocardium. This modifies the old model of distinct la...

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