  After the morphological classification of the 18,190 $^{12}$CO molecular
clouds, we further investigate the properties of their internal molecular gas
structures traced by the $^{13}$CO($J=$ 1$-$0) line emissions. Using three
different methods to extract the $^{13}$CO gas structures within each $^{12}$CO
cloud, we find that $\sim$ 15$\%$ of $^{12}$CO clouds (2851) have $^{13}$CO gas
structures and these $^{12}$CO clouds contribute about 93$\%$ of the total
integrated flux of $^{12}$CO emission. In each of 2851 $^{12}$CO clouds with
$^{13}$CO gas structures, the $^{13}$CO emission area generally does not exceed
70$\%$ of the $^{12}$CO emission area, and the $^{13}$CO integrated flux does
not exceed 20$\%$ of the $^{12}$CO integrated flux. We reveal a strong
correlation between the velocity-integrated intensities of $^{12}$CO lines and
those of $^{13}$CO lines in both $^{12}$CO and $^{13}$CO emission regions. This
indicates the H$_{2}$ column densities of molecular clouds are crucial for the
$^{13}$CO lines emission. After linking the $^{13}$CO structure detection rates
of the 18,190 $^{12}$CO molecular clouds to their morphologies, i.e.
nonfilaments and filaments, we find that the $^{13}$CO gas structures are
primarily detected in the $^{12}$CO clouds with filamentary morphologies.
Moreover, these filaments tend to harbor more than one $^{13}$CO structure.
That demonstrates filaments not only have larger spatial scales, but also have
more molecular gas structures traced by $^{13}$CO lines, i.e. the local gas
density enhancements. Our results favor the turbulent compression scenario for
filament formation, in which dynamical compression of turbulent flows induces
the local density enhancements. The nonfilaments tend to be in the low-pressure
and quiescent turbulent environments of the diffuse interstellar medium.
