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. 1This studv was suvDorted in Dart bv the Cardiovascular Training Proj$am U.