Complex sentence: The common iliac arteries are two large arteries that originate from the aortic bifurcation at the level of the fourth lumbar vertebra and bifurcate into the external iliac artery and internal iliac artery.
Split into simpler sentences: The common iliac arteries are two large arteries that originate from the aortic bifurcation at the level of the fourth lumbar vertebra. They bifurcate into the external iliac artery and internal iliac artery.