Evidence of Upper Cervical Asymmetry
- The Venter posterior musculi digastrici goes from the mastoid process behind the atlas process and exerts a forward pressure onto it, before going to the hyoid bone (see left image); on the right side, the muscle goes in front of the atlas process to the hyoid bone without impinging on the atlas process (see right image)
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.