THE MEMBRANOUS PART OF THE HUMAN INTERVENTRICULAR CARDIAC SEPTUM

The arrangement of collagen and elastic fibers of the membranous part of the interventricular septum (PMS) was studied in hearts from adult humans. Connective bundles formed a network of fairly independent tend ons arranged in two layers. The tendinous bundles consisted essentiall y of type I collagen fibers while type III fibers were visible as a th in network with transversely and longitudinally oriented meshes around the muscle bundles. Cranial and caudal to the PMS were narrow and irr egular bands of collagen fibers that apparently represented zones of l ow resistance to the high blood pressures acting from the left to the right heart chambers. The predominance of fiber bundles arranged in an approximately transverse direction with regard to the arterial cone a xis suggests a resistance to enlargement resulting from high aortic bl ood pressure. Elastic fibers were observed in the transitional zone be tween the cardiac muscle and the PMS. They were continuous with elauni n fibers and these with oxytalan fibers closely intermingled with the narrow network of type I collagen fibers of the PMS. The successive tr ansformation of elastic fibers, which were very numerous in the muscle -tendon transition, into elaunin and these into oxytalan fibers toward the central portions of the PMS suggests a functional sequence charac terized by a high elasticity and consequent mobility of the transition region itself and by a progressive increase of resistance in this por tion.