DIVISION OF THE HEART AND AORTIC BULB. 879 
and behind the pericardium by the fusion of the adjacent ends of the anterior and posterior 
parts of the anterior ventral aorte respectively. 
The simple tubular heart, at first straight and of fairly uniform calibre, soon alters in 
form and in the relative position of its different parts. It becomes irregularly enlarged, 
and a series of four dilatations, with intervening constrictions, can be distinguished. The 
‘dilatations, from behind forwards, are as follows: (1) the sinus venosus or saccus 
-reuniens, (2) the auricle or atrium, (3) the ventricle, and (4) the aortic bulb or bulbus 
arteriosus. The short constriction between the auricle and ventricle is known as the 
auricular canal, and the less pronounced constriction which intervenes between the 
ventricle and the aortic bulb is termed the fretum Halleri. In addition to this alteration 
‘in form, the tubular heart elongates, much more so than the pericardium in which it lies, 
whilst at the same time the anterior extremity of the aortic bulb and the sinus venosus 
are withdrawn, from the root of the neck and the septum transversum respectively, into 
the pericardium. Of necessity, therefore, the single heart is bent upon itself, and it 
projects more and more into the pericardium, pushing forwards the visceral layer of 
the serous lining of this cavity, and carrying with it a mesentery of splanchnic mesoderm 
which is known as the mesocardium. 
The bending of the heart results in the formation of a U-shaped loop, the posterior or 
venous limb of the loop lying to the left and above, the anterior or arterial limb being to 
the right and below, whilst the intervening stem of the loop runs from the left and above 
downwards, forwards, and to the right. The apex of the aortic bulb is bent a little to the 
left and reaches the middle line. 
Subsequently the auricle ascends behind the ventricle, and the ventricular opening 
of the auricular canal, the short communicating passage between the auricle and 
ventricle, is seen as a transverse slit at the upper part of the left or posterior end of the 
ventricle. As the ventricular chamber enlarges its anterior end passes towards the 
middle line, and the ventricle is no longer so obliquely directed from left to right but 
lies more in the mesial plane. As the result of these changes of position, and the coin- 
cident modifications in size of the different parts, the aortic bulb is eventually placed 
immediately in front of the auricle, and the opening of the auricular canal is nearer 
the middle of the upper part of the posterior wall of the ventricular chamber. During 
its further growth the ventricle enlarges principally at its lower or ventral part; the 
dorsal part is not materially altered in position, and consequently the openings of the 
auricular canal and the aortic bulb remain relatively at the same level. The auricle 
increases in size by lateral expansion and by forward extension of its lateral angles ; 
the forward extensions embrace the sides of the aortic bulb, and constitute the rudi- 
mentary auricular appendages. 
DIVISION OF THE HEART INTO ITS DIFFERENT CHAMBERS, AND DIVISION OF 
THE AORTIC BULB. 
Whilst the changes in form, position, and size of the different sections of the primitive 
heart, which have just been described, are taking place, the division of the heart cavity 
into its four permanent chambers, and of the aortic bulb into its aortic and pulmonary 
portions commences. These divisions are brought about by the growth of septa in the 
ventricle, auricle, and aortic bulb, and by the thickening and fusion of the middle portions 
of the upper and lower walls of the auricular canal. 
The thickenings of the walls of the auricular canal are called endocardial cushions ; 
they meet and fuse together mesially to form the septum intermedium by which the 
central portion of the canal is obliterated, whilst the lateral portions are left patent as 
small triangular channels which still connect the auricular and ventricular chambers. 
The separation of the ventricular part of the heart into right and left chambers is 
indicated, externally, at a very early period, by a groove, well marked in front and below, 
but less distinct behind where it runs upwards to the auricular canal; whilst internally, 
in a corresponding position, a ventricular septum (s. inferius) grows upwards from the 
inferior part of the ventricular wall. The posterior part of the upper border of this septum 
unites with the lower end of the fused endocardial cushions of the auricular canal and the 
dorsal wall of the ventricle between the auricular and aortic bulb orifices; the anterior 
part of its upper border terminates a short distance below the orifice of the aortic bulb, 
where it unites with the septum of the aortic bulb which, at a later period, descends to 
meet it. 
The division of the aortic bulb commences at its distal end between the orifices of 
the fourth and fifth cephalic aortic arches. It is due to the ingrowth of two endocardial 
is 
