THE DEVELOPMENT OF THE HEART IN MAN. 
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The right ventricle considerably exceeds in the anterior view the left ventricle, and 
its upper part, widely dilated, leads to the pulmonary stem, which lies horizontally. 
The atria are of relatively enormous size, enclosing the ventricular portion on 
each side, are of irregular outline, and are apparently in diastole ; while the small size 
of the cavity of the ventricles and their thick walls point to the condition of systole 
of the ventricles. The wall of the left ventricle is very considerably thicker than 
that of the right ventricle. 
Internal Structure. — The Interior of the Right Atrium . — The auricular portion of 
this cavity — large, wide, and with trabeculae of muscle tissue upon its wall — forms by 
far the greater part of this cavity. The right venous valve forms a large, almost 
vertical partition, stretching from the dorsal wall along the floor to the anterior wall 
and shutting off a small cavity on its left side from the large cavity on the right; 
there being only a small, rounded orifice between the anterior concave margin of the 
central part of this valve on the one hand, and the anterior wall of the atrium on 
the other hand (text-figs. 14, 15, and 16). 
The anterior attachment of the base of the right venous valve is split into two 
TRANS. ROY. SOC. EDIN., VOL. LII, PART II (NO. 12). 45 
