THE DEVELOPMENT OF THE HEART IN MAN. 
299 . 
6. The Atrio-ventricular Junction, the Formation of the Atrio-ventricular 
Valves and of the Conducting Mechanism. 
The' atrial canal is at first a short channel lying. on the left side of the heart, but 
it moves to the right and comes to lie centrally, and it is no longer visible from 
the surface but is concealed by the overlapping atria and ventricles. 
The upper and lower cushions of the canal fuse with one another centrally, and 
.the lateral margins of the channel remain as the venous ostia. All these changes 
are exactly as are shown in Born’s models and figures. 
The orifices are at first destitute of valves, but soon rudiments of valves appear 
on both sides, right and left ; the first to appear being a large laterally placed cusp 
formed by the infolding of the atrio-ventricular junction into the interior, and by its 
undermining on the ventricular side. 
This cusp represents, on the right side, the lateral and anterior cusps of the 
tricuspid valve, and on the left side the lateral cusp of the mitral valve. 
This cusp formation, by infolding, has an important bearing upon the loss of 
continuity between the muscle walls of the atrium and the ventricles. 
The structure of the infolded cusp is a centrally placed wedge-shaped mass of 
connective tissue derived from the external surface of the atrio-ventricular junction, 
with a layer of muscle tissue continuous with the atrial muscle coat on the atrial 
surface, and of ventricular muscle substance on the ventricular aspect. On the atrial 
surface there may, in addition, be small masses of cushion tissue. 
At the, margin of the cusp the two layers of- muscle tissue are continuous with 
one another. As the tissue of the cusp becomes altered into fibrous tissue this 
continuity is destroyed. 
The right lateral cusp is of large size, and it is attached to the ventricular wall 
by muscle bands of considerable size. Orally it gains an attachment to the infun- 
dibulum, close to the region of the septum which divided this channel from the aorta, 
and this attachment as a rule remains fibrous and can be readily identified in the 
adult heart. 
At each orifice later a second cusp is formed, the medial cusp being in each case 
derived from the central mass of tissue representing the fused cushions of the atrial 
canal, and from the adjacent muscle tissue of the septum of the ventricles. 
The endocardial cushions of the atrial canal form a central mass of considerable 
size, connected dorsally with the atrium, and especially with the atrial septum, and 
ventrally with the ventricle muscle tissue. ^ 
The cushion mass extends widely on either side, and on the right side a prolonga- 
tion of the upper cushion extends above the right venous ostium and joins with the 
bulbar cushion B. 
On the left side, the upper and lower cushions send prolongations to the ventricle 
muscle wall above and below the left venous ostium. 
