VESSELS AND NERVES OF THE HEART 



519 



During systole, as a result of this arrangement: — (1) The papillary muscles and the longi- 

 tudinal and antero-posterior axes of the ventricles are simultaneously shortened. (2) There is 

 a movement of torsion or "wringing" which reduces the ventricular cavities to their minimum 

 dimensions. 



Conducting system. — 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 atrio-ventricular bundle, and serves to transmit the atrial rhythm of contraction to the 

 ventricles. 



The atrio-ventricular bundle begins in the septal wall of the atrium a short distance in 

 front of the coronary orifices (fig. 428). It has an expanded free end, the atrio-ventricular 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 millimetres 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 supraventricul- 

 aris and above the papillary muscle of ths conus, giving oif branches to the latter and to other 

 small papillaries on the septum (fig. 428). 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 hmb of the bundle appears in the left ventricle a little below the septum mem- 

 branaceum. It is a wide band immediately beneath the endocardium, which cannot usually be 



Fig. 432. — Diagram of one Anterior 

 AND ONE Posterior Superficial Bundle 

 OF Cardiac Muscle Fibres seen from 

 Behind. (After MacCallum.) 



Fig. 433. — ^Diagram op a Deeper 

 Bundle of Muscle Fibres. (After 

 MacCallum.) 



Conus arteriosus 



Lower anterior 

 superficial 

 »^. - ""^kT^ bundle 



Left anterior 

 papillary 

 muscle 



Anterior superficial 

 bundle 



Conus arteriosus 



— Tendon of the conus 

 Right atrio-ven- 

 tricular ring 



stripped off without injuring the bundle (fig. 429). 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 trabeculaj, 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 fibres which form a sub-endocardial plexus. This plexus, visible to the 

 naked eye (p. 516) is made up of fibres having a structure similar to those of the ventricular 

 portion of the bundle. The fibres 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 

 recognised 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 terminaUs of the right atrium. Una- 

 nimity is stiU lacking with regard to the physiological significance of this structure. 



Vessels and Nerves of the Heart 



The arteries. — The two coronary arteries arise from the right and left sinuses of the aorta. 



The right coronary artery [a. coronaria dextra] passes forward between the pulmonary 

 artery and the right atrium, and then follows the right coronary sulcus to the diaphragmatic 

 surface of the heart (fig. 435), to anastomose with the left coronary artery. The posterior 

 descending branch [ramus descendens posterior] arises at the posterior longitudinal sulcus. It 



* Arch. f. Anat., Physiol, u. wissenschafthche Medizin. 



t Das Reitungssystem des Saiigertierherzens, Fischer, Jena, 1906. 



