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PROFESSOR D. WATERSTON ON 
More orally, its margin is free, and above it sweeps above the level of the inter- 
ventricular foramen and is connected to the bulbar septum, as described later. 
The muscle wall of the ventricle is trabecular, and many of the strands are under- 
mined. The anterior papillary muscle is foreshadowed as a thick strand of muscle 
incompletely separated from the wall of the ventricle, united at one end to the 
ventricular surface of the cusp, and at the other attached to the lateral wall of the 
ventricle. Its base is connected to the septum by a short, stout band of muscle. 
The “tricuspid” valve at this stage possesses but a single cusp, representing the 
anterior and posterior cusps of the adult heart. Its most oral part gives rise to the 
tendinous cord attaching the anterior cusp to the bulbar septum. 
On the left side the structures are simpler both in the valve cusps and the 
papillary muscles. Medial and lateral cusps are present— the medial a thick rounded 
projection from the central endocardial cushions attached to a prominent muscle 
bundle in the floor of the ventricle, and orally prolonged on to the lateral wall. 
The lateral cusp overhangs into the cavity from the atrio-ventricular junction, and 
it extends to both the anterior and posterior papillary muscles. 
Right Ventricle . — The tubular portion lies even more horizontally than that in the 
former specimen, and it is flattened from side to side. The proximal end has widened 
out and extends towards the right ventricle. A short partial spiral groove, distinct 
on the right side, marks off a more dorsal and vertical portion from the ventral 
oblique portion. 
The accompanying figure (text-fig. ll) shows the extent of the septum dividing 
