Page 52 - Clinical Anatomy

ECA1 7/18/06 6:31 PM Page 37

The mediastinum 37

however, extend low enough for this new septum to overlap the foramen
secundum in the septum primum and hence to close it.
The two overlapping defects in the septa form the valve-like foramen
ovale which shunts blood from the right to left heart in the fetus (see ‘fetal
circulation’ below). After birth, this foramen usually becomes completely
fused leaving only the fossa ovalis on the septal wall of the right atrium as
its memorial. In about 10% of adult subjects, however, a probe can still be
insinuated through an anatomically patent, although functionally sealed
foramen.
Division of the ventricle is commenced by the upgrowth of a fleshy
septum from the apex of the heart towards the endocardial cushions. This
stops short of dividing the ventricle completely and thus it has an upper
free border, forming a temporary interventricular foramen. At the same
time, the single truncus arteriosus is divided into aorta and pulmonary
trunk by a spiral septum (hence the spiral relations of these two vessels),
which grows downwards to the ventricle and fuses accurately with the
upper free border of the ventricular septum. This contributes the small pars
membranacea septi, which completes the separation of the ventricle in such a
way that blood on the left of the septum flows into the aorta and on the right
into the pulmonary trunk.
The primitive sinus venosus absorbs into the right atrium so that the
venae cavae draining into the sinus come to open separately into this
atrium. The smooth-walled part of the adult atrium represents the contri-
bution of the sinus venosus, the pectinate part represents the portion
derived from the primitive atrium.
Rather similarly, the adult left atrium has a double origin. The original
single pulmonary venous trunk entering the left atrium becomes absorbed
into it, and donates the smooth-walled part of this chamber with the pul-
monary veins entering as four separate openings; the trabeculated part of
the definitive left atrium is the remains of the original atrial wall.

The development of the aortic arches
and their derivatives (Fig. 31)
Emerging from the bulbus cordis is a common arterial trunk termed the
truncus arteriosus, from which arise six pairs of aortic arches, equivalent to
the arteries supplying the gill clefts of the fish. These arteries curve dorsally
around the pharynx on either side and join to form two longitudinally
placed dorsal aortae which fuse distally into the descending aorta.
The 1st and 2nd arches disappear; the 3rd arches become the
carotids. The 4th arch on the right becomes the brachiocephalic and right
subclavian artery; on the left, it differentiates into the definitive aortic arch,
gives off the left subclavian artery and links up distally with the descending
aorta.
The 5th arch artery is rudimentary and disappears.
When the truncus arteriosus splits longitudinally to form the ascending
aorta and pulmonary trunk, the 6th arch, unlike the others, remains linked
with the latter and forms the right and left pulmonary arteries. On the left