opening 



from sinus venosus 



1 St arch 



truncus shown in outline over atria 



interatrial septum 

 atrioventricular cushion 



eptum II 

 valvulae venosae / /ept^f" I 



ridges in truncus separating 

 pulmonary and systematic openings 



Interatrial fenestra 11 {ostium II) 

 nterotriol fenestra I (ostium 1) 



ght ventricular chamber 



left ventricular chamber 



base of truncus 

 A 3.7 mm interventricular septum 



sinus venosus opening 



B 



atrioventricular canal 



trabeculae carnae 



atrioventricular cushion 



foramen ovale 



anterior vena cava 

 postcaval stem 



interatrial septum 

 atrioventricular cushion 



coronoid 



bundle of His 



pulmonary arch 



systemic arch 



nterventricular septum 



6 mm 



bicuspid (mitral) valve 



semilunar valves of 

 pulmonary arch 



8.8 mm 



tricuspid valves 

 interventricular septum 



bicuspid valve 



Figure 11-1. Development of the mammalian heart. A, semidiagrammatic frontal section of the 

 heart of a 3.7-mm pig; B, anterior holf of A as seen from behind; C, 6-mm pig; D, anterior half of 

 C; E, 9.4-mm pig; F, anterior half of E; G, 8.8-mm humon; H, anterior half of G. (After Patten, 1 946) 



between the openings into the fourth and sixth aortic arches. 

 These ridges are extended backward along the truncus wall 

 in a clockwise course of just over one-half turn. The right 

 lateral ridge comes to lie left ventrally in the conus opening, 

 and the left ridge is right dorsal in position. The edges of these 

 extend outward until they meet, thus dividing the truncus. 

 The fused ridges then join the interventricular septum which 

 has also fused with the atrioventricular cushion to complete 

 the separation of the ventricles. 



Concurrently the sinus venosus is drawn into the wall of 

 the right auricle until the now formed anterior and poste- 

 rior venae cavae open separately, the former in front of the 

 latter. (The development of these vessels will be described 

 later in this chapter.) Then a coronary vessel opens behind 

 the posterior. The sinus tissue is represented by the sinu- 

 atrial node of the adult heart. 



The second interatrial septum develops until only a small 

 foramen ovale remains. This foramen is not in line with the 



osteum II of the primary interatrial septum. The cushions 

 margining the openings between the atria and ventricles 

 now begin to develop as flaps supported by chordae tendi- 

 neae and papillary muscles. These braces for the valves are 

 sculpted out of the thick muscular walls of the ventricle. 

 Valves develop from the cushions at the base of the pulmo- 

 nary and systemic trunks, three in each of these. The pulmo- 

 nary drainage has appeared and has entered the left auricle. 

 These pulmonary openings appear as new developments; 

 their origin from the original venous entry into the right 

 atrium is not indicated. 



Among mammals, the heart appears to be constant in its 

 structure, varying only in such details as the pattern and 

 points of origin or exit of the coronary vessels. Of functional 

 interest is the fact that some of the heart muscle tissue has 

 been modified into a conducting system for coordinating the 

 action of the heart. This system is formed of a bundle of His 

 and Purkinje fibers, and it includes the sinu-atrial node in 



340 . THE CIRCULATORY SYSTEM 



