i 5 i8 



HANDBOOK OF PHVSIOI.I « :\ 



CIRCULATION II 



lions of normal and pathologic channels have 

 further advanced and clarified the interarterial and 

 transarterial communications and their branches. 1 



The Myocardium 



ventricles. Gross dissection studies (108, 153, 232) 

 reveal a rather consistent and orderly arrangement in 

 mammals with quantitative differences overshadowed 

 by qualitative similarities. Every muscle fascicle 

 originates from the fibrous rings at the base, the super- 

 ficial fibers descending toward and penetrating the 

 apex to form the vortex spirals, and then looping 

 upward as the deeper fibers which ascend along the 

 endocardial surface to reinsert in the annulus fibrosis. 

 Thus, the two ventricles are encompassed by figure- 

 of-eight bands of muscle with origins and insertions 

 at the base and a fulcrum at the apex. The muscular 

 interventricular septum receives part of these fascicles 

 while an intermediate layer encircles only the left 

 ventricle, also adding to the septum. 



There are probably no true cleavage planes between 

 isolated fascicles but, rather, the ventricle represents 

 a single muscle mass dividing and branching into 

 intercommunicating fascicles. In any one plane, how- 

 ever, the fibers are more or less constant, the epicardial 

 fibers running perpendicular to the endocardial 

 fibers at any given point. 



atria. Nearly all fibers arise and insert into the A-V 

 rings, but some fibers merge and disappear on the 

 muscular coats of the great veins. 



Interatrial fibers form the septal areas while the 

 auricles and pectinate regions are largely intra-atrial 

 fascicles. There are two simple layers — an inner 

 horizontal and an outer vertical — bound by much 

 intertwining interdigitation. 



1 :i >\ducting system. Commencing with the sino-atrial 

 node at the superior vena cava and right atrium, 

 specialized myomeric conducting tissue traverses the 

 right side of the interatrial septum to the locus of the 

 atrioventricular node (153, 399). The latter is situated 

 on the atrial side of the base of the tricuspid valves' 

 medial leaflet, above the coronary sinus and between 



1 No attempt has been made to give a complete bibliography 

 which would involve consideration of many thousands of publi- 

 cations. Except for an occasional lead article, the older work is 

 considered by referring to some 40 to 50 reviews, monographs, 

 and symposia. Direct but incomplete reference is made to the 

 more recent work not covered in such summaries. By this 

 means, most of the important work in the field can be found by 

 the interested reader although direct reference may no! In- 

 made to it. 



the limbus fossa ovalis and the medial leaflet. From 

 the A-V node, the bundle of His penetrates the fibrous 

 A-V ring and runs in the posterior membranous 

 interventricular septum, branching into the right and 

 left bundles at this site or in the upper muscular 

 septum. The right bundle branch is solitary in its 

 course through the septum to the base of the modera- 

 tor band, while the left bundle subdivides into many 

 branches. The terminations of each bundle form many 

 fine fasciculi intimately applied to the endocardium 

 before merging with the contractile myocardium. 



Coronary Arteries 



The course and distribution of the major coronary 

 arteries in all mammalian subgroups is remarkably 

 similar and intergroup differences are less pronounced 

 than intragroup variations. The basic anatomic pat- 

 terns are thus comparable from the smallest to the 

 largest mammals, i.e., from rodents to whales (63, 

 78, 142, 298). 



There are two coronary arteries, right and left, 

 arising respectively from the right anterior and left 

 anterior aortic sinuses of Valsalva. The ostia are 

 situated above the reflections of the semilunar valves, 

 the right coronary in man being 35 ° to the right, and 

 the left coronary 65 to the left of the anteroposterior 

 axis of the body (258). 



left coronary artery. This vessel courses in epi- 

 cardial areolar tissue anteriorly and to the left in the 

 auriculoventricular groove, between the pulmonary 

 artery and the left auricular appendage, and bifur- 

 cates into the anterior descending and circumflex 

 branches (fig. 1). These two branches are quite con- 

 stant in all species, the bifurcation occurring 1 to 1.5 

 cm (84, 1 89-191, 258) from the ostium in man, and 

 2-4 mm in dogs and smaller mammals (36, 64, 153, 

 290). In dogs and rabbits, but not in man, monkeys, 

 or higher primates (63, 64, 78, 172, 173, 189, 258), a 

 septal artery arises just prior to, at, or not uncom- 

 monly, just beyond the bifurcation on the descendens 

 or circumflex, in that order. Small branches from the 

 left coronary artery are frequently present passing to 

 the pulmonary conns and left atrium, and in the 

 rabbit, branches from both left and right coronary 

 arteries supply the major portion of the vasa vasorum 

 of the pulmonary artery (351). A third primary 

 division has also been described arising between the 

 above and supplying the anterior left ventricle (348). 

 The anterior descendens follows the anterior inter- 

 ventricular sulcus toward the apex and is of variable 

 length, terminating prior to, at, or beyond the apex. 



