DEVELOPMENT OF THE HUMAN HEART 273 



inferior septum behind (the right posterior tip, projects well 

 into the interventricular foramen, while the right anterior tip 

 blends with the septum aorto pulmonale. It becomes clear, by 

 comparing this stage with older ones, as well as with the adult 

 (figs. 15 to 18), that this common central mass is destined to pro- 

 duce the medial valves of the two venous ostia, namely anterior 

 cusp of the mitral valve and the medial cusp of the tricuspid valve. 



But in addition to the anterior and posterior endocardial cush- 

 ions there are also two lateral cushions well demonstrated by 

 His and by Greil. One of these, the left lateral, is first seen in an 

 embryo 9 mm. (No. 422) long, (fig. 7), but both are not well pro- 

 nounced until the embryo is 11 mm. long (No. 353, figs. 11 and 

 16). At this time a lateral' process from the septum aorto pul- 

 monale reaches to and blends with .the right lateral cushion. The 

 left lateral is still sharply defined and isolated. The line of con- 

 nection between the right lateral cushion and the septum is always 

 marked in the adult by a tendon which passes into the aortic sep- 

 tum or by a small papillary muscle which holds the same position. 

 This muscle is called medial papillary muscle by Henle. 20 Between 

 the tendon or muscle and below the anterior tip of the medial 

 cusp there is alway a clear field, the bottom of which is formed by 

 the membranous septum. 



Semi-diagrammatic reconstructions of the tendinous masses 

 at the base of the heart are shown in figs. 15 to 18. Fig. 15 is 

 from the heart of embryo No. 163 (9 mm.) with the left lateral 

 cushion from No. 422 (9 mm.) added. The muscular wall of the 

 atrial canal is indicated and the connective tissue is stippled. 

 Fig. 16 is from embryo No. 353 (11 mm.) which shows that the 

 origin of the aorta has shifted well into the left ventricle. In 

 fig. 17, No. 296 (17 mm.) the sinuses marking the three semi- 

 lunar valves are indicated upon the two wings of the septum aorto 

 pulmonale, and upon the anterior endocardial cushion. Fig. 

 18 shows the arrangement of the connective tissue at the base of 

 the adult heart. The parts of the valves which are formed by 



20 Henle, Gefasslehre, IIAuflage, 1876, fig. 33. This tendon is constant, in fact 

 the most constant of all of the attachments of the valves in the right ventricle. 

 It is pictured in all anatomies, but is not recognized in the B.N. A. 



