Fiber bundle direction in the mammalian heart. An extension of the "nested shells" model

The orientation of the muscle fiber bundles within the mammalian left ventricle was examined in a variety of mammals. The hearts were arrested in situ in animals with an intact thorax by means of an isotonic K+ solution perfused via the aorta and coronaries. The hearts were then fixed by formalin perfusion through the same vessels and the hearts embedded in gelatin. Serial sections were prepared perpendicular to the Apex-Valve axis. On close examination, the muscle fibers show the change in orientation from endocardium to epicardium previously described by others. In addition, the clefts and voids of the inner one-third to inner one-half of the left ventricular wall add another dimension to the fiber direction: the fiber bundles appear to take a curving course from the middle of the wall to the endocardial surface. This pattern was visible in all studied hearts. Speculations are made on the significance of this anatomic arrangement.