I imagine the major vertical structural members run through those for and aft semi-circular cavities. any cross pieces would most likely be between the decks themselves, so the outer walls would just be a skin attached to minor crosspieces. Either way, you're not looking at much in the way of hull thickness and if you want to take Shaw's sketches at face value then that black outer line probably scales to something like six inches anyway.
I think it's almost
pointless to discuss the structural framework of the TOS ship, since the ship's overall geometry -- though visually appealing -- defies common sense when it comes to a sound structural design. Ask almost anyone who's built a model kit of the thing.