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\label{spine}  All the evidence presented in this paper, taken together, strongly suggests that Nessie forms a spine-like feature that runs down the center of the Scutum-Centaurus Arm of the Milky Way. How did it get there? Is it the crest of a classic spiral density wave \citep{Lin1964}, or does it have some other cause? Any feature this long and skinny that is not controlled by Galactic-scale forces will be subject to a variety of instabilities, and cannot last long. It would be great if we could look to numerical simulations for answers, but today's simulations can, alas, only give hints. Nessie is {\it so} skinny, and {\it so} much denser than its surroundings that no extant numerical simulation has the combination of spatial resolution and dynamic range in density needed to produce a feature like it.  Figure \ref{fig:simulation} offers a snapshot(available as a movie at \url{http://empslocal.ex.ac.uk/people/staff/cld214/movies.html})  of a numerical simulation \cite{2013MNRAS.432..653D} that represents the state of the art at present. present (available as a movie at \url{http://empslocal.ex.ac.uk/people/staff/cld214/movies.html}).  One can see density features that are highly elongated, both within the spiral arms, and also between the arms. Many of the features between the arms in Figure \ref{fig:simulation} are similar to the `spurs' and `feathers' that have been simulated and observed by E. Ostriker and colleagues \citep{Shetty2006,Vigne2008,Corder2008}. Figure \ref{fig:IC342} (discussed below) shows a recent WISE image of the galaxy IC342 \citep{Jarrett2013}, and it is clear from that image that some `spiral' galaxies also exhibit inter-arm filaments that are even more pronounced than the simulated spurs and feathers. In the case of Nessie, the velocity information analyzed in \S \ref{CO} and \ref{ammonia} seems to very strongly favor Nessie's being oriented exactly along (within, as the backbone of) an arm (Scutum-Centaurus) over the idea that Nessie is a spur or interam filament.