278 FRANKLIN P. MALL 



extends into the atrial part of the valves, which at first is continu- 

 ous through the tendinous cords with the ventricular muscle. At 

 first the atrio-ventricular muscle connection is through the main 

 wall of the ventricle, but as this is resolved into the trabecular 

 system with the growth of the valves and the formation of the 

 papillary muscles the connection between atrium and ventricle 

 is through the tendinous cords which are at first muscular. Later 

 with the degeneration of the muscle in the cords the muscular 

 connection between the atria and ventricles was believed to have 

 been broken down completely. In lower vertebrates the mus- 

 cular connection between the atria and ventricles through the 

 trabecular system remained throughout life, and the significance 

 of this has been fully demonstrated by Gaskell 23 and His, Jr. 24 

 In man, however, all of the cords are converted into connective 

 tissue, but the muscle of the atria extends well into the valves or it 

 may reach to its free border (Kiirchner). 25 It has also been 

 stated that in some rare instances the tendinous cords of the mitral 

 valve remain muscular in the adult (Oehl). 26 But the muscular 

 connection through the valves is first interrupted at the free thin 

 edges and later the muscular fibers in the tendinous cords dis- 

 appear. The break in the muscular connection at the free edges 

 of the valves will be considered in discussing the atrio-ventricular 

 bundle. 



Returning to the description of embryo No. 353 (11 mm.), 

 it is possible to determine with considerable precision the various 

 adult connections of the valves. The most definite valve is the 

 anterior cusp of the mitral which is formed by the union of the 

 left lateral tips of the anterior and posterior endocardial cushions. 

 Each tip is here bound to the trabecular system by well formed 

 muscle strands, one of which passes to the anterior wall of the 

 heart and the other to the posterior as well as to the septum. It 

 is clear that these two strands, which appear much earlier than in 



23 Gaskell, On the innervation of the heart with special reference to the heart of 

 the tortoise. Jour. Phys., vol. 4, 1884. 



24 \V. His, Jr., Die Thatikeit des embryonalen Herzens. Arbeiten aus der med, 

 Klinik zu Leipzig, F. C. W. Vogel, 1893. 



25 Kiirchner, Wagner's Handworterbuch, vol. 2, 1844. 



26 Oehl, Henle's Anatomie, Gefiisslehre, Braunschweig, 1876. 



