and I have touched the sky."
Is that from some well-known author, or just made for the ep title.
Well I am really reaching back, but I seem to recall that the old person they encounter who the rest of the civilization regards as a kook says the line. So I guess I assumed it was created for the show.
Not the origin but maybe of interest.
http://www.poetsforum.com/poems/p_hollow.html
Believe it or not, it seems to be original to Star Trek.
My former roommate and I used to call that episode “For my head is hollow, and I have touched myself.”
Just sayin’.
sounds like Harlen Ellison
That’s exactly it. I have most of TOS on tape (sadly, I have Spock’s Brain but not The Enterprise Incident) and saw this ep only a few weeks ago while I was ill in bed. The old geezer (we don’t really get any info about what the rest of the civ think of him) dares to pipe up the truth to Kirk & co., knowing he is sentencing himself to death but old enough not to care (*), and the phrase “…for the world is hollow, and I have touched the sky” is the last thing he gets out.
It is vaguely reminiscent of Harry Harrison’s Captive Universe, but as Harry’s book wasn’t published until TOS was history, any plagiarism can’t be laid at Roddenberry’s door.
(* They already know; and the geezer knows that they have come from outside the world. It was just something he wanted to get off his chest before he died.)
the trope is from letters of Emily Dickinson: “knowing the world is hollow. . .”
I’d be surprised if the ST:TOS writer was aware of it, but ever since I first saw the Flammarion Engravinga few decades ago, I’ve connected the two in my mind.
Crappy ep…our heroes suddenly cant fight worth shit* and worse of all, it just looks bad. Typical third season ep.
*Yes I know McCoy has space-cancer.
I am like 90% it is actually said in the episode. Episode titles were more poetic in TOS especially compared to TNG where nearly every title was, “The (Noun)”
Better you should buy “Turnabout Intruder” instead. At least then you can watch Shatner chew up the scenery. 
In my days as a Trekkie, before home video, I watched the local indy channel showing of ST (there was only one in those days) without fail, for years. I saw every episode at least three times, some many more.
I saw “Turnabout Intruder” once, and only remember scattered bits from it. Still have never seen it in remembered entirety.
FWIW, I consciously avoided saying I remembered only snatches of it.
Instead of a whiney dimwit who’s a slave to her hormones, it features a viscious psychopath who’s a slave to her hormones. 
Yes, it’s said in the episode. But the question is whether it is also a reference to something else. It’s just so poetic that people question whether the Trek writers could have actually come up with it themselves.
I know that the first “poetic” titles I thought of beyond this was “Who Mourns for Adonais,” and it is definitely a quote. “Who mourns for Adonais?” is the 413th line of Percy Bysshe Shelley’s pastoral poem, “Adonaïs: An Elegy on the Death of John Keats, Author of Endymion, Hyperion, etc.”