Conducting system fo the heart
Although the ordinary myocardium of the atria is distinct from that of the ventricles there is, at one place, a connection between them. This connection is by means of a small band of muscle which differs histologically from ordinary heart muscle. It is known as the atrioventricular bundle, and serves to transmit the atrial rhythm of contraction to the ventricles.
The atrioventricular bundle begins in the septal wall of the atrium a short distance in front of the coronary orifices. It has an expanded free end, the atrioventricular node, from which branches pass to be quickly lost in the atrial myocardium. The bundle passes forward covered by endocardium and by one or two millimeters of myocardium, and passes beneath the medial cusp of the tricuspid valve. In passing from the atrium to the ventricle, the bundle skirts the lower margin of the septum membranaceum. Immediately in front of the septum membranaceum it divides into a left and right limb, of which the former pierces the muscular interventricular septum. The right limb now passes beneath the crista supra ventricularis and above the papillary muscle of the conus, giving off branches to the latter and to other small papillaries on the septum. Bending somewhat toward the apex, it enters the moderator band which conducts it to the large anterior papillary muscle. From here it passes along one of the trabeculae connected with the sterno-costal wall of the ventricle, or in the wall itself, to reach the posterior papillary muscle or muscles. The right limb is compact and rounded and in the intact heart is usually invisible except, sometimes near the root of the moderator band or in the band itself.
The left limb of the bundle appears in the left ventricle a little below the septum membranaceum. It is a wide band immediately beneath the endocardium, which cannot usually be stripped off without injuring the bundle. It passes along the septal wall toward the apex and divides into two parts, which again subdivide to be distributed to the anterior and the posterior papillary muscles. The branches for the papillary muscles may reach them through thick trabecula; or they may form thin strands which, covered only by endocardium, bridge from septum to papillary muscle.
In addition to the comparatively distinct branches to the papillary muscles of both ventricles, the bundle gives off finer fibers which form a sub-endocardial plexus. This plexus, visible to the naked eye is made up of fibers having a structure similar to those of the ventricular portion of the bundle. The fibers were described by Purkinje as long ago as 1845,* but it was not until 1906, thirteen years after the discovery of the bundle by W. His, Jr., that Tawaraf recognized their significance.
There is another node of muscle having characters similar to that of the conducting system, although not connected with it except by myocardium of the ordinary character. This is the sinus-node which is situated at the upper end of the crista terminalis of the right atrium. Unanimity is still lacking with regard to the physiological significance of this structure.
* Arch. f. Anat., Physiol, u. wissenschafthche Medizin.
t Das Reitungssystem des Saiigertierherzens, Fischer, Jena, 1906.