The posterior arch of a cervical vertebra forms about two-fifths of the circumference of the ring: it ends behind in the posterior tubercle, which is the rudiment of a spinous process and gives origin to the Recti capitis posteriores minores. The diminutive size of this process prevents any interference with the movements between the atlas and the skull. [ http://en.wikipedia.org/wiki/Posterior_tubercle_of_cervical_vertebra ]