Top: ¹³CO(2–1) (in white) and C¹⁸O(2–1) (in black) spectra overlayed on the Herschel column density map of Musca (Cox et al. 2016) for the northern position. The offset is centred on: α₍₂₀₀₀₎ = 12^h 28^m58^s and δ₍₂₀₀₀₎ = –71°16′55′′, and 100′′ corresponds to ~0.07 pc at a distance of 140 pc. C¹⁸O emission is only detected at the crest (green-yellow-red), while ¹³CO can be used to trace the strands (blue) as well. Bottom right: several ¹³CO(2–1) spectra are displayed that were extracted at the indicated positions in the map above. This shows that ¹³CO has two components towards the filament crest. Bottom left: same for C¹⁸O(2–1), demonstrating it is only detected towards the filament crest.