The vertebrae are a series of twenty-four hollow roughly cubical bones that form the spine and protect the spinal cord. They are stacked one on top of another and reach from the skull to the pelvis. They are separated by cartilaginous discs which allow the spine to be flexible and cushion any compression in the spinal column, particularly when humans stand upright.
All mammals have twenty-four vertebrae. In humans, they are of more or less identical size and shape, although those towards the head are smaller. However, in other mammals, vertebrae can be of surprisingly different construction. For example, the giraffe has exactly the same number of vertebrae in the neck as humans do. In animals with a tail, the tail is usually made up of one extended vertebrae, which is the vestigal tailbone or coccyx in humans.
When performing a lumbar puncture, the physician must find a space between the vertebrae in order to insert the needle. This must also be done while avoiding the spinal cord. As a result, the site for the puncture is usually in the upper back.