THE DEVELOPMENT OF THE HEART IN MAN. 
267 
orifices appear on each side of it. At this level a transverse area of closely packed, 
deeply staining nuclei runs across the mass of cushion tissue, but probably this area 
may indicate the line or junction of the upper and lower cushions. This area can be 
traced from the dorsal surface of the cushion, where it begins in the floor of the 
atrium, onwards to the apex of the interventricular septum, and hence corresponds to 
the region of the atrio-ventricular node and bundle. 
At the external atrio-ventricular groove on the surface of the heart there is a 
very distinct region of loose connective tissue, intervening partially between the 
atrial and the ventricular musculature. 
The lower endocardial cushion, between the right and left atrio-ventricular 
orifices, lies against the base of the septum primum, and is continued below the 
septum primum in the floor of the atrium to reach the margin of the orifice of the 
vena cava inferior, and is prolonged also towards the wall of the coronary sinus.. 
On the ventricular side it passes on to the interventricular septum below the 
interventricular opening. To the right it blends with bulbar cushion B below the 
right atrio-ventricular orifice, and to the left it merges into the trabecular musculature 
of the left ventricle (text-fig. 5). 
The right ventricle is triangular in shape and is smaller than the left, but has a 
larger cavity and thinner walls. 
The muscle wall is irregularly reticulated. 
The cavity is narrow, and passes gradually into the cavity of the distal portion. 
The displacement of the atrio-ventricular orifice towards the right, so that its 
