288 HANDBOOK OF PHYSIOLOGY ^^ CIRCULATION I 



FIG. I. Diagrammatic view of heart showing cavities of 

 both ventricles and atria. At upper left, superior and inferior 

 venae cavae may be seen entering left atrium. Black ellipse in this 

 area indicates region of sinoatrial node. Atrioventricular node 

 is indicated above tricuspid valve cusps in interatrial septum. 

 Right bundle passes down into right ventricle; the two branches 

 of left bundle pass into left ventricle. Numerous Purkinje 

 branches cross ventricular cavity on both sides. [From Scher 

 (114a).] 



FIG. 2. The conduction system as seen from the right and left 

 sides. In A: right bundle terminates near anterior papillary 

 muscle on the right. In B.- the two branches of left bundle 

 pass to general regions of anterior and posterior papillary 

 muscles on the left. Much of the Purkinje tissue lining the en- 

 docardium is not represented. [From Scher (114a).] 



ventricles, in the various mammalian forms which 

 have been studied. 



It is common (129), although not strictly correct, 

 to refer to all of these cells as "Purkinje cells." Prop- 

 erly, this term should be restricted to those cells 

 which distribute the impulse in the ventricles, and 

 which were originally described by Purkinje (93). 

 The reasons why the term Purkinje fibers is extended 

 to other cells are that some of the anatomical char- 

 acteristics overlap and that the A-V nodal cells are 

 directly connected to the Purkinje fibers of the 

 common bundle. Furthermore, since Purkinje's 

 description, various anatomists have found cells, 

 which they believe to be Purkinje fibers, at a large 

 number of locations in both the atria and the ven- 

 tricles, and in a variety of hearts. These findings have 

 somewhat weakened the specificity of the description 

 of these cells. 



Myocardial Cells 



Ordinary heart muscle cells are striated like skeletal 

 muscle. Their diameter ranges from 15 to 25/11. The 

 length is not definitely known since available electron 

 micrographs do not show both ends of a cell; length 

 is probably 150/1. Within the fibers or trabeculae are 

 slightly oval nuclei placed on transverse sections. The 

 cytoplasm shows cross striations. The sarcolemma 

 surrounding; the nucleus contains the contractile 



fibrils. Submicroscopic details as well as the syncytial 

 organization of the cells have been mentioned in 

 Chapter 1 1 . 



Pacemaker Cells 



The sinoauricular node, the pacemaker of the 

 heart, was discovered by Keith & Flack (63) who 

 noted tissue, similar to that previously discovered by 

 Tawara (127) in the atrioventricular node, at the 

 junction of the superior vena cava and auricle. A 

 portion of their description follows: 



'Tn the human heart the fibres are striated, 

 fusiform, with well-marked elongated nuclei, plexi- 

 form in arrangement, and embedded in densely 

 packed connective tissue — in fact, of closely similar 

 structure to the Knoten [referring to the auriculo- 

 ventricular node]. The special neuromuscular system 

 lies at the junction of the free border of the appendix 

 with the mouth of the superior cava, and extends 

 downwards along the sulcus terminalis for a distance 

 of about 2 cm in man. In tliickness it is approxi- 

 mately 2 mm. The muscular fibres are small, being 

 but a half or third the breadth of those of auricular 

 fibres proper." 



Pacemaker cells are found in the sinus venosus of 

 the vertebrate heart and it appears that similar 

 pacemakers exist in the hearts of all species (36, 64, 

 67, 86, 123). The pacemaker cells are, as indicated, 

 about lo/i in diameter in inan or the dog. 



The S-A node, like the A-\' node, is riclily supplied 

 with nerve endings and ganglion cells. As has been 

 mentioned, the sinus node is the pacemaker of the 

 heart and the electrical signs of cardiac activity for 

 each beat commence in the node and travel thence 

 over the remainder of the atrium. 



The Atria: Atrial Conduction System 



It is generally felt that (with the exception of the 

 region around the A-\' node which will be discussed 

 below) atrial muscle is not composed of more than 

 one functional type of cell. One source of contention 

 is the presence or absence of an atrial conduction 

 system or, more generally, of Purkinje fibers distrib- 

 uted within the atria. The atria of birds, fish, reptiles, 

 and mammals have been studied. In the first of these 

 a well-developed atrial conduction system has been 

 described (35, 78, 86). Arguments have been ad- 

 vanced and some evidence presented that an extensive 

 atrial conduction system exists, or that there are 

 Purkinje cells, in the atrium in the other forms. The 

 literature on this subject is surprisingly extensive and 



