THE DEVELOPMENT OF THE HEART IN MAN. 
263 
the dorsal and ventral walls. It does not reach the ventral portion of the floor of 
the atrium, and there is present the foramen ovale primum, measuring in the model 
some 20 mm. in vertical height (text-fig. 3). 
In the upper part of the septum primum there is a small orifice with ragged 
edges — the foramen ovale secundum. 
The septum primum terminates caudally and ventrally in the cushion tissue of 
the atrio-ventricular canal. The right portion of the atrium is much larger than the 
left, and forms a very large chamber lying to the right of the septum primum. 
Between the septum primum and the sinu-atrial orifice is 
intersepto-valvular space. 
a distinct recess, the 
Table II. 
Dimensions of Heart of SI. x 100 
V ertical height . 
Atrium, transverse width . 
„ vertical height ..... 
Sinu-atrial orifice, height . ... . 
Distance between right and left venous ostia 
Ventricle, left, vertical height . . . . . 
„ „ transverse width . . 
„ „ thickness of wall . . . 12 
,, right, transverse width 
,, „ thickness of wall ... 7 
T , , - , r f dorso-ventrally 
Interventricular foramen I J 
l vertically .... 
Distance of interventricular foramen from interventricular cleft 
170 mm. 
150 „ 
150 „ 
25 „ 
40 „ 
115 „ 
50 „ 
to 18 „ 
60 „ 
to 12 „ 
21 „ 
30 „ 
45 „ 
Atrial Canal . — The exit from atrium to ventricle is now divided into two 
minute channels by the union with one another of the endocardial cushions of the 
atrial canal. 
