On the left side, the posterior digastric muscle pulls behind the upper cervicals and exerts a functionally and morphologically effective pressure on them. However, on the right side, the digastric muscle runs in front of the upper cervicals. Analogous to a pulley, the left posterior digastric muscle lifts the left atlas extension before attaching to the hyoid bone. On the right, this is not so: There the digastric muscle runs in front of the atlas extension to the hyoid bone. This results in an asymmetry of the atlas in relation to over-&-underlying structures with marked limitation of head movement.
This is probably the only asymmetrically oriented pair of muscles within an organism with generally paired and symmetrically oriented muscles.