systemic arch In the embryo of a tetrapod, the fourth aortic arch which, in the adult, becomes the main source of the blood supply to all parts of the body other than the head. In amphibians and reptiles both right and left arches are present in adults. In birds only the right arch is present in adults and in mammals only the left arch is present in adults.

systemic arch A paired blood vessel in the embryos of tetrapods that carries blood from the aorta to the trunk and hind limbs. It is derived from the fourth aortic arch. Adult amphibians and reptiles retain both arches, while birds and mammals only have one.