Remember my pig head? At long last (due more to busy schedules than speed of dermestid beetle handiwork) my baby porker was recently delivered back - sparkly clean, completely odorless, and very much fun to investigate. It's obvious looking back at the photo of the skull still covered in flesh, but I didn't actually make note at the time that the head included the first few vertebrae of the neck - the atlas and the axis. I first learned about this highly specialized pair of bones from an artist's anatomy book during a college drawing class and remember being intrigued by their form, function, and mellifluous names. Of course all the bones in any given skeleton are exquisitely constructed to make the animal bodies they support as efficient in behavior and environment as possible, but the atlas and axis of vertebrates, working together as a joint*, have the special task of determining the exact range of motion of the head, while simultaneously protecting the brainstem where it exits the skull - both rather critical responsibilities.
Named after the Greek god who in myth bore the full weight of the world on his shoulders, the atlas holds up the mass of the head and establishes the connection between the skull and the long chain of stacked bones that form the spine. The block of vertebrae making up the neck are known as cervical vertebrae, so the atlas is scientifically abbreviated as C1. Whether an animal is bipedal and stands upright, or has a horizontal back and stands on four legs, the head is supported, and linked to the neck, by the atlas. Where the occipital bone (back) of the skull and the atlas meet, the atlanto-occipital joint occurs, allowing a rocking motion that moves the head up and down. Peculiar to this vertebra is that, unlike all others, it has no "body" and is instead primarily just a ring, through which the brain stem passes, and the top of the axis fits. To support and nod the head, the atlas has significant lateral masses that anchor many muscles and ligaments.
The axis (C2), as it's name implies, creates a pivoting joint with the atlas by fitting neatly into C1 with a nobby protrusion called the odontoid process. Around the odontoid process, the atlas can rotate, allowing the head to turn and shake; this joint is known as the atlanto-axial joint and is the only pivoting joint in the body. Below is an anterior (frontal) view of the atlas and axis of my porker - showing the odontoid process notching up through the atlas - and a posterior view of the atlas and axis positioned at the back of the skull. Pig cervical anatomy isn't identical to that of a human, but the general form and function of these 2 bones is closely shared among land-based vertebrates. This youtube clip, though actually in reference to cervical spine disks, begins with a great 3D rendered animation of the cervical vertebrae in humans, and the unique motion of the atlanto-axial joint. And I gotta include this clip as well, from Dr. Tory Robson of the Woodbury Spine & Injury Center; in it, he gives a good overview of typical, healthy cervical spine and neck anatomy, as they appear in radiograph, then he moves on to discusses a rather awful case from a car accident injury which resulted in a twisted log jam of bones (and presumably a challenging project for a spinal surgeon). The anatomy lesson here is nice, but the props go entirely to the charismatic Dr. Tory and his inspired video direction (the commentary is also amazing)... the whole thing, it like blew my mind...
*In some species of whale and dolphin the atlas and axis (and sometimes other cervical vertebrae) are fused to provide more stability for powerful swimming in their streamlined ocean environment.
**As it pertains to the pull-quote - the few exceptions to the seven cervical vertebrae rule of most mammals are the manatee (with six), the two-toed sloth (also six), and the three-toed sloth (with a whopping nine).