The Gill Arches:
The gill pouches are later lost, the remaining associated vessels being remnants of the early pattern (Figure 2.07A). The internal carotid artery, right and left, is the remnant of gill arch 3. The arch of the aorta is the remnant of the right 4th arch, while the right subclavian artery is the remnant of the left 4th arch.
The ductus arteriosus, a shunt between pulmonary artery and aorta, is a remnant of the 6th left arch. The 5th arch is entirely lost. The 1st arch forms the maxillary arteries and parts of the external carotid arteries. The 2nd arch forms part of the stapedial arteries.
The path of entry of blood into the early heart depends on where the blood has come from. If it has come from the placenta and lower body most of it will pass directly through the foramen ovale and into the left atrium (Figure 2.07B). Blood from the head and upper body mainly passes into the right atrium.
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