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PROFESSOR D. WATERSTON ON 
Within this cushion are shown very clearly strands of closely packed, deeply 
staining nuclei, which traverse the loose general tissue of the cushion and extend 
from beneath the lumen of the atrium downwards and forwards towards the ventricle. 
This upper cushion blends on the right with the bulbar cushion B, both above and 
below the atrio-ventricular orifice. . To. the left it blends with the trabecular tissue 
in the wall of the ventricle, ventrally with the interventricular septum, and on the 
surface of this septum there is seen exactly the same arrangement of the bulbar 
cushion A as was seen in Bl. 
The lower cushion makes its appearance first to the left of the right atrio- 
ventricular orifice, where it forms a round tubercle, and the line of junction of the 
upper and lower cushions runs obliquely from right to left and caudally. There is 
a similar extension of this lower cushion across the floor of the atrium to the dorsal 
wall, and also similar strand-like arrangements of nuclei within its substance. This 
cushion forms the right margin of the right atrio-ventricular orifice, and below that 
orifice its left extremity is attached to the trabecular musculature of the ventricles. 
It descends for some little distance on the left side of the interventricular septum. 
The extent and connections of these cushions are shown in the frontal reconstruc- 
tion, text-fig. 7. 
The interior of the cylindrical portion of the right ventricle is shown in Plate- 
fig. 8. 
In the proximal portion the lumen is undivided, and the prominent bulbar 
cushion A projects into it. More distally the lumen is divided into two, and the 
bulbar cushion is prolonged into each as a projection on the interior. 
