THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 563 



there are formed from the interstitial connective-tissue substance 

 tendinous plates, which with the endocardial cushions attached 

 to their margins become the permanent atrioventricular valves 

 (tig. 311 B mk). The latter therefore arise from a part of the 

 spongy wall of the ventricle. 



The remnants of the shrivelled muscular trabeculse (fig, 311 B cht), 

 which are attached to the valve from below, become still more 

 rudimentary in the immediate vicinity of the attachment : here also 

 a part of the muscular fibres disappears entirely ; the connective 

 tissue, on the contrary, is preserved, and is converted into the tendinous 

 cords which, known under the name of chordce tendinece, serve to 

 hold in place the valves. At some distance from the latter the 

 trabeculse projecting into the ventricle preserve their fleshy con- 

 dition and become the papillary muscles (pm), from the apices of 

 which the chordae tendinese arise. "Whatever of the primitive 

 trabecular network still persists on the inner surface of the ventricle 

 forms a more or less stout mesh work of muscles, the fleshy pillars of 

 the heart (tc), or trabeculse carneae." 



In consequence of all these alterations the originally small cavity 

 of the ventricle has become considerably enlarged at the expense of 

 a part of its spongy wall. For the whole of the space which in 

 fig. 311 B lies below the valves has been produced from the system 

 of originally narrow spaces (fig. 311 A), and has been employed for the 

 enlargement of the central cavity by the degeneration of the fleshy 

 columns into slender tendinous cords. 



It still remains for us to investigate the division of the truncus 

 arteriosus and the final metamorphosis of the atrium. 



At about the time when the formation of the partition in the 

 ventricle takes place, the truncus arteriosus, which arises from it, 

 becomes somewhat flattened, and thus acquires a fissure-like lumen. 

 On the flat sides two ridge-like thickenings make their appearance 

 '(fig. 310-4 and B s), grow toward each other, and by their fusion 

 divide the cavity into two passages which are triangular in cross 

 section. Now, too, the beginning of the internal separation makes 

 itself visible externally as two longitudinal furrows, in the same 

 way that the formation of a partition in the ventricle is indicated 

 by the sulcus interventricularis. The two canals resulting from the 

 division are the aorta and the pulmonary artery (Ao and Pu). For 

 a time they continue to be surrounded by a common adventitia, then 

 they become widely separated and also externally detached from each 

 other. The whole process of separation in the truncus arteriosus 



