Why do the first 2 cervical vertebrae on a horse have a different name? Do they move differently?
Lindsey's answer: The first two cervical vertebrae, the atlas (also C-1) is the very top and sits on the axis (also C-2), work together to allow movement. Then the others are labeled or named C-3 through 7. They're larger to help with the size of the head and allow nodding type movement. When a horse is 'flexing at the poll' they're actually using these two vertebrae. So if there's a misalignment it can be painful for the horse.
Works the same in people and the same in dogs, etc. Same names, too. In fact, the two vertebrae are the same for all mammals (I think all, same for all I'm familiar with). There are seven cervical vertebrae in all mammals except manatees who have 6 and sloths who have an extra. The only thing that changes is the size of the vertebrae - think cat versus horse, same number but huge size difference!