THE DEVELOPMENT OF THE HEART IN MAN. 
273 
Atrio-ventricular Junction . — The ostia from the right and left divisions of the 
atrium into the corresponding ventricles are large, and around these orifices there is 
a definite though rudimentary arrangement of valves. The right ostium is a long 
vertical slit, 50 mm. long and 12-15 mm. in width ; while the left is slightly smaller 
but is wider, measuring some 30 mm. by 20 mm. The right ostium is bounded on 
the right by a large flap formed by an infolding of the tissues into the interior of 
the ventricle. 
Similarly in the left ventricle there is an early “mitral” valve, the lateral cusp 
formed by an infolding from the left side, while the medial (aortic) cusp is derived 
from the cushion tissue between the left and right atrio-ventricular orifices. 
The cusps of the valves are attached by a network of muscle tissue to the 
ventricular wall. Some bands are specially distinct. In the right ventricle one 
sweeps from the lateral cusp already described on to the interventricular septum, 
and it is clearly derived from the similar band seen in Bl. 
There is an advance in the condition in Bl, but the differences are not so great 
as to prevent the recognition of corresponding structures. 
The transverse width of the muscular wall of the atrial canal is, in this specimen, 
180 mm. 
At the right venous ostium, the atrio-ventricular junction is infolded into the 
interior of the ventricle so as to form a large curved flap, undermined and concave 
on the ventricular aspect, extending from the caudal border of the ostium along its 
right side to its cephalic border and on to the interventricular septum. In the 
caudal portion this flap forms a cusp of some thickness and is attached to the inter- 
ventricular septum (text-figs. 9 and 10). 
TRANS. ROY. SOC. EDIN., VOL. LII, PART II (NO. 12). 
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