I THE HEART 537 



they form a considerable portion of the moderator band and also appear as gelatinous-looking 

 strands on the inner walls of the atria and ventricles. They also occur in the human heart asso- 

 ciated with the terminal distributions of the bundle of JHis^ The fibers are very much larger in 

 size than the cardiac cells and differ from them in several ways. In longitudinal section they are 

 quadrilateral in shape, being about twice as long as they are broad. The central portion of each 

 fiber contains one or more nuclei and is made up of granular protoplasm, with no indication of 

 striations, while the peripheral portion is clear and has distinct transverse striations. The fibers 

 are intimately connected with each other, possess no definite sarcolemma, and do not branch. 



The muscular structure of the heart consists of bands of fibers, which present an exceedingly 

 intricate interlacement. They comprise (a) the fibers of the atria, (6) the fibers of the ventricles, 

 and (c) the atrioventricular bundle of His. 



The fibers of the atria are arranged in two layers a superficial, common to both cavities, and 

 a deep, proper to each. The superficial fibers are most distinct on the front of the atria, across 

 the bases of which they run in a transverse direction, forming a thin and incomplete layer. Some 

 of these fibers run into the atrial septum. The deep fibers consist of looped and annular fibers. 

 The looped fibers pass upward over each atrium, being attached by their two extremities to the 

 corresponding atrioventricular ring, in front and behind. The annular fibers surround the auriculae, 

 and form annular bands around the terminations of the veins and around the fossa ovalis. 



The fibers of the ventricles are arranged in a complex manner, and various accounts have 

 been given of their course and connections; the following description is based on the work of 

 McCallum. 1 They consist of superficial and deep layers, all of which, with the exception of 

 two, are inserted into the papillary muscles of the ventricles. The superficial layers consist 

 of the following: (a) Fibers which spring from the tendon of the conus arteriosus and sweep 

 downward and toward the left across the anterior longitudinal sulcus and around the apex of 

 the heart, where they pass upward and inward to terminate in the papillary muscles of the left 

 ventricle; those arising from the upper half of the tendon of the conus arteriosus pass to the 

 anterior papillary muscle, those from the lower half to the posterior papillary muscle and the 

 papillary muscles of the septum. (6) Fibers which arise from the right atrioventricular ring and 

 run diagonally across the diaphragmatic surface of the right ventricle and around its right border 

 on to its costosternal surface, where they dip beneath the fibers just described, and, crossing the 

 anterior longitudinal sulcus, wind around the apex of the heart and end in the posterior papillary 

 muscle of the left ventricle, (c) Fibers which spring from the left atrioventricular ring, and, 

 crossing the posterior longitudinal sulcus, pass successively into the right ventricle and end in 

 its papillary muscles. The deep layers are three in number; they arise in the papillary muscles 

 of one ventricle and, curving in an S-shaped manner, turn in at the longitudinal sulcus and end 

 in the papillary muscles of the other ventricle. The layer which is most superficial in the right 

 ventricle lies next the lumen of the left, and vice versa. Those of the first layer almost encircle 

 the right ventricle and, crossing in the septum to the left, unite with the superficial fibers from 

 the right atrioventricular ring to form the posterior papillary muscle. Those of the second 

 layer have a less extensive course in the wall of the right ventricle, and a correspondingly greater 

 course in the left, where they join with the superficial fibers from the anterior half of the tendon 

 of the conus arteriosus to form the papillary muscles of the septum. Those of the third layer 

 pass almost entirely around the left ventricle and unite with the superficial fibers from the lower 

 half of the tendon of the conus arteriosus to form the anterior papillary muscle. Besides the 

 layers just described there are two bands which do not end in papillary muscles. One springs 

 from the right atrioventricular ring and crosses in the atrioventricular septum; it then encircles 

 the deep layers of the left ventricle and ends in the left atrioventricular ring. The second band 

 is apparently confined to the left ventricle; it is attached to the left atrioventricular ring, and 

 encircles the portion of the ventricle adjacent to the aortic orifice. 



The atrioventricular bundle of His (Fig. 501), is the only direct muscular connection known to 

 exist between the atria and the ventricles. Its cells differ from ordinary cardiac muscle cells in 

 being more spindle-shaped. They are, moreover, more loosely arranged and have a richer vascu- 

 lar supply than the rest of the heart muscle. It arises in connection with two small collections of 

 spindle-shaped cells, the sinoatrial and atrioventricular nodes. The sinoatrial node is situated on 

 the anterior border of the opening of the superior vena cava; from its strands of fusiform fibers run 

 under the endocardium of the wall of the atrium to the atrioventricular node. The atrioventricular 

 node lies near the orifice of the coronary sinus in the annular and septal fibers of the right atrium; 

 from it the atrioventricular bundle passes forward in the lower part of the membranous septum, 

 and divides into right and left fasciculi. These run down in the right and left ventricles, one on 

 either side of the ventricular septum, covered by endocardium. In the lower parts of the ventricles 

 they break up into numerous strands which end in the papillary muscles and in the ventricular 

 muscle generally. The greater portion of the atrioventricular bundle consists of narrow, somewhat 

 fusiform fibers, but its terminal strands are composed of Purkinje fibers. 



1 Johns Hopkins Hospital Reports, vo 



