DEVELOPMENT OF THE HUMAN HEART 281 



valve is bound to the ventricle by anterior and posterior papil- 

 lary muscles. In the course of time the valves and papillary 

 muscles will be renamed to correspond with their development, 

 which will also accord with the adult condition. 27 



C. THE ATRIOVENTRICULAR BUNDLE 



No complete description can be given to the formation of the 

 tricuspid and mitral valves without considering the fate of the 

 muscular wall of the atrial canal. This at once defines definitely 

 the atrio-ventricular bundle which is embryologically the rem- 

 nant of the auricular canal after the greater portion of its muscular 

 wall has been destroyed by the formation of the valves. 



His, Sr., described the breaking down of the muscular wall of 

 the atrial canal in small embryos and states that it is nearly broken 

 down in an embryo 13.8 (?) mm. long. Later His, Jr., showed 

 the presence of a muscle bundle connecting atrium with ventricle 

 in the new-born child and discusses the connections between 

 atrium and ventricle in lower animals as well as in the embryo. 

 He noted that the character of the heart beat changed in the chick 

 at the time the atrial canal is forming, at the time the muscular 

 connection between the atria and ventricles is reducing. 28 It is 

 easy to read into this paper that His thought that the muscular 

 connection between the atria and ventricles never does break down 

 completely, although he never states it. Were there a complete 

 separation of the muscle wall of the atria and ventricles before the 

 formation of the atrio-ventricular bundle is formed heart block 

 should occur in the embryo. That the atrio-ventricular bundle 

 is a remnant of the primary connection in the embryo is mentioned 

 by Tandler as a possibility but he believes it improbable. He is 



27 Direct dissection of the heart of an embryo 30 mm. long (No. 90, a) shows that 

 the medial valves of the two ostia form a saddle hanging over the ventricular 

 septum. Instead of single lateral cushions double lateral cushions are present on 

 both sides, the posterior being larger than the anterior on the right side. A simi- 

 lar condition is sometimes seen in serial sections. In case the ventricles are cut 

 t hrough transversely the valve system can be beautifully seen from below, and this 

 method of investigation will probably settle this question definitely. 



28 His, Jr., I.e., p. 19. 



