Left recurrent laryngeal nerve has a longer course because of which arch aery:

The Left recurrent laryngeal nerve has a longer course to the neck than the right side. This is because it hooks under the left sixth arch aery which persists in extra-uterine life as the ductus aeriosus, a fibrous remnant. On the right side, neither the sixth nor fifth arch aeries persist and so the recurrent laryngeal nerve is restrained by the next most superior structure which is the fouh branchial arch aery (Right Subclan Aery). Rarely, the right fouh branchial arch aery does not develop in the adult and instead, the right dorsal aoa of the fetus persists to supply the same territories. Hence, the right recurrent laryngeal nerve is no longer restrained by a subclan aery and so it divides from the vagus more superiorly to run inferiorly on the larynx. This anomalous nerve (Non Recurrent Laryngeal nerve) may be transected during thyroid surgery. * The recurrent laryngeal nerve has differing courses oneach side. On the right, it loops under the subclanaery and on the left under the aoic arch. On bothsides it then ascends on the side of the trachea.