DEVELOPMENT OF THE HUMAN HEART 279 



this specimen and continue throughout development, are the 

 anlagesof the anterior and posterior papillary muscles respectively. 

 In addition a heavy strand of tissue encircles the lateral side of 

 the left ostium and unites the apices of the two papillary muscles. 

 In the middle of this, protruding into the left ostium, is seen the 

 conspicuous left lateral cushion already noted in an embryo 

 9 mm. long (No. 422, fig. 7). This arrangement corresponds with 

 what is found in the adult heart. Each papillary muscle not only 

 attaches itself to the tips of the anterior cusp but also to the tips 

 of the posterior cusp. (It is confusing to use the B.N.A. terms 

 anterior and posterior in naming the cusps when the embryological 

 terms should be medial and lateral.) Between the two papillary 

 muscles there is often a third or lateral papillary muscle, or nu- 

 merous small muscles or the two muscles may be widened to fill 

 this area. At any rate the structures found around the left ostium 

 in embryo 353 represent fully what is present in the adult. We 

 have in all cases two papillary muscles, each of which communi- 

 cates freely with the tips of the two cusps of the mitral valves. 

 On the dorsal side and in front of the left ostium the muscle of the 

 atrium and ventricle is continuous, as is shown in fig. 16. 



As the heart grows larger more chordae tendinea are formed 

 necessarily nearer to the base of the heart. It follows that the 

 primary condition found in No. 353 represents only the tips of the 

 valves and that their subsequent enlargement is due partly to 

 stretching of the anlage and partly to the undermining of the wall 

 of the ventricle. 



The united anterior and posterior endocardial cushions were 

 projected at first into the left ventricle but subsequently its left 

 tip becomes lodged in the interventricular foramen (fig. 11). 

 To this smaller portion the septum aorto pulmonale attaches 

 itself anteriorly and the inferior muscular septum posteriorly. 

 The left half hangs freely within the left ventricle throughout life, 

 forming a loose flap or sail which is suspended between the left 

 venous ostium and the vestibule of the aorta. While the semi- 

 lunar valves of the aorta are somewhat distant from this flap in 

 embryo No. 353, later on they are pushed down to it so that the 

 aorta becomes finally attached to its base (fig. 17). 



