deletions | additions       diff --git a/sectionIntroduction_.tex b/sectionIntroduction_.tex  index 74e688d..ee140f9 100644  --- a/sectionIntroduction_.tex  +++ b/sectionIntroduction_.tex 
...
\section{Introduction}  Many surprisingly fundamental questions remain about the structure of the Milky Way. For instance, does the Milky Way have two \cite{Jackson_2008,Francis_2009, Dobbs_2012} or four \cite{Reid_2009, Bobylev_2013,Urquhart_2013} major spiral arms? What is the precise location of these arms in position-position-velocity space? And what is the nature of inter-arm structure---is it made of well-defined spurs or more web-like structures? An understanding of the Milky Way's true structure has eluded us, largely due to the fact that we are embedded in the galaxy we are attempting to delineate.  Much of our current understanding of the Milky Way's three-dimensional structure stems from radial velocity measurements of gas. Making use of the Milky Way's rotation curve \cite{McClure_Griffiths_2007, Reid_2014}, we can translate line-of-sight velocities into distances, thereby constructing a gross three-dimensional model of our Galaxy. Thanks to a wealth of spectroscopic surveys, velocity-resolved observations are readily available for much of the Galaxy's molecular, atomic, and ionized gas. Extended tracers, like CO \citep{Dame_2001} or HI \citep{Shane_1972} provide the best constraints on the Galaxy's overall anatomy. To probe finer structure, observations of high-mass star forming regions can also provide kinematic and distance information for high-density gas. For instance, measurements of trigonometric parallaxes and proper motions of masers from the BeSSeL survey produced accurate locations for several spiral arm segments, along with their associated pitch angles \citep{Reid_2014}. Likewise, the Bolocam Galactic Plane Survey \citep[BGPS,][]{Schlingman_2011, Shirley_2013}, Shirley_2013, Ellsworth_Bowers_2013},  the Millimetre Astronomy Legacy Team 90 GHz Survey \citep[MALT90,][]{Foster_2011,Jackson_2013}, and the $\textrm{H}_2\textrm{O}$ Southern Galactic Plane Survey \citep[HOPS,][]{Purcell_2012,Walsh_2011} have produced hundreds of high-spectral resolution velocity measurements of the dense gas in molecular clouds. Analyses of extinction data from surveys like Pan-STARRS1 complement this emission line data and can also be used to create three-dimensional models of the Galaxy's structure \citep{Green_2014,Schlafly_2014}. While the tools available for probing the Milky Way's internal structure are diverse, none has especially high three-dimensional resolution over wide areas. To address this problem, \citet{Goodman_2014} recently suggested that extraordinarily elongated filamentary infrared dark clouds, termed "bones" could be used to determine the structure of the Milky Way's spiral arms, their location within the Galaxy, and their appearance as viewed by an outside observer. \citet{Goodman_2014} presented Nessie as the first "bone" of the Milky Way. They found that Nessie was at least three degrees ($\sim 140$ pc), and possibly as long as eight degrees ($\sim 450$ pc) in length, while being less than 0.1 deg ($\sim 0.6$ pc) wide. They also conclude that Nessie lies very near the geometric mid-plane of the Milky Way Galaxy, at the 3.1 kpc distance to the Scutum-Centaurus arm. An analysis of the radial velocities of ${\rm NH}_3$ emission and CO emission confirms that Nessie runs along the Scutum-Centaurus arm in {\it p-p-v} space, suggesting it forms a dense spine of that arm in physical space as well \citep{Goodman_2014}