As Shaw pointed out, the bridge alcove diagram is of an earlier version of the ship. Do not place too much stock in it.
While it is admittedly simplified (which is totally fine with me if this is just a panel to show possible damage like the
screens in ST II), it is blatantly obvious that the
three vertical structures (i.e. saucer, dorsal, nacelle pylons) perfectly match three of the vertical structures in the Jefferies cross-section in TMOST.
Therefore it stands to reason that the exact same vertical structures in the (black & white) cross-section serve the same function, or am I imagining things?
As for why there are no horizontal turbo paths evident on the cross section in TMOST? The answer is simple: none of them fall along the centerline of the ship.
A feasible rationalization, but, again, the top view of the bridge alcove schematic reveals the absence of horizontal ("turbo shaft") structures, while all the vertical ones and the diagonal lines to the nacelles are highlighted in a different color.
Also, from a practical point of view I'm confident that Matt Jefferies would have had a turbo shaft running from the top of the dorsal all the way down to the bottom of the engineering.
And why is allegedly reversed L-shaped "turbo shaft" in the dorsal (not to mention the strange dorsal feature it circumvents) going up to the exterior center structure at the stern of the saucer?!?
Considering that Jefferies himself felt the warp nacelles to be extremely dangerous there are probably lethal radiation levels that would also severely contaminate any turbo lift car going there.
If the fourth and bigger vertical structure (going all the way down to the bottom of the engineering hull) is a turbo shaft, apparently in close proximity to the yellow circle at the underside of the engineering hull, than the yellow circle is the cover hatch for the waste dump of such contaminated turbo lift cars?
To assume these vertical structures to be turbo shafts doesn't seem to make a lot of sense bearing the bridge schematic in mind and the actual turbo lift entry points and movement featured in the series. YMMV.
Bob