THE DEVELOPMENT OF THE HEART IN MAN. 
287 
The developmental changes which occur in the heart subsequent to the earliest 
stage described here may conveniently be discussed in the following groups : — 
1. Changes in the external form of the. heart. 
2. Changes affecting the sinu-atrial junction. 
3. The division of the atrium. 
4. The atrial canal and the bulbus cordis. 
5. Changes in the primitive bulbus cordis. 
6. The atrio-ventricular junction, the formation of the atrio-ventricular valves 
and of the conducting mechanism. 
1. Developmental Changes in the External Form of the Heart. 
The changes in the external form of the heart indicate successive changes in the 
size and position of the various segments of the primitive heart tube. 
Atrium. — The atrium is from the first placed dorsally, and extends across the 
whole transverse width of the heart. Its cephalic surface is rounded and only 
slightly indented centrally by the truncus arteriosus. The atrium is everywhere 
separated by a deep, groove from the ventricle and from the bulbus cordis. 
Throughout it retains this dorsal position. Wing-like expansions grow out on either 
side, curving round and partly enclosing the bulbus and, later, the pulmonary artery 
and the aorta. 
After the formation of the septum primum, the atria increase greatly in their 
relative vertical depth, but the region of the attachment of the primitive septum 
grows less rapidly, and hence an infolding of the wall is produced in that region. 
Ventricle. — The ventricular portion of the heart consists at first of a Y-shaped 
loop, with right and left limbs, separated from one another, except at the apex, by 
an oblique groove, the bulbo-ventricular groove. These limbs at a still earlier stage 
lie — the one cephalic, the other caudal — in position, and the intervening groove is 
horizontal ; but a gradual rotation occurs, so that the groove becomes oblique and 
eventually vertical in position. The bulbo-ventricular groove extends nearly to the 
opposite margin of the heart, and its termination lies opposite to a slight notch 
upon that border. 
The left limb of the ventricle is at first triangular in outline, but later becomes 
more spherical in shape and forms the left ventricle. The right portion becomes 
flask-shaped, the caudal portion being spherical and the oral tubular in form. 
The right and left ventricles are from the first clearly demarcated from one another 
by the groove upon the ventral surface and by the notch upon the caudal margin of 
the connecting piece. The notch may persist and be present even in the adult heart. 
The right ventricle, at first placed with its long axis vertical, becomes slightly 
oblique from right to left orally, and the distal portion slopes more and more 
obliquely in the dorsal direction. The deep cleft between the oral portion of the two 
