Orientation of the fascicles of the ventricular myocardium

A new technique was developed to examine the arrangement of the fascicles of the myocardium. Liquid plastic was injected intramurally into the free walls and the septum of more than 300 hearts of mammals and birds. After the plastic hardened, the tissues were digested in alkali, thereby releasing a plastic cast which had been formed in the myocardium. The plastic plates formed in the heart were consistently perpendicular to the surfaces of the epicardium and the endocardium. Related studies have shown that each cluster or clone of muscle fibers is contained in a fibrous connective tissue capsule. The injected plastic is therefore confined to the trabecular (extracapsular) clefts which define the borders of the capsules. The casts show that the capsules containing the muscle fibers are arranged in the planes of great circles of the myocardial spheroid. These capsules are shaped as wedges with acute angles at the endocardium, and with dihedral angles less than 1 degree. Interventricular sulci interrupt and separate the capsules of the right free wall from the left free wall. Septal capsules are separated from but aligned with the long directions of the capsules of the adjacent free walls. These data challenge the currently accepted concept of the bandage-like arrangement of the myocardial fascicles. The significance of our findings with respect to myocardial function is discussed.