I am still at a loss as to the rank structure and if it was based upon the level of duties assigned or not. It does not seemed to have been strickly based upon the levels of duties as assigned, since a Brigade surgeon was not relieved of his duties as the regimental surgeon of his regiment in which he was originally assigned.
It would seem that the higher up the chain of command that one went depended upon the level of medical experience that a Surgeon had, but that the rank that a surgeon held was not based upon that same experience. It other words a Brigade or Division chief surgeon may have still only been a Captain in rank, where you would have normally expected a staff position at that level would normally expected to had been a Major's rank.