The backbone is made of series of bones (vertebrae) stacked one over the other with spines projecting backwards. The spinal cord and nerves are enclosed within the backbone. Take a look close look at the area normally used to perform epidural or spinal in the figure below.

The spinal cord is enclosed in a sac containing fluid (spinal fluid in spinal space). The sac is surrounded by vertebrae. The space surrounding the sac and inside the vertebrae is the epidural space. The nerves come out of the spinal cord and traverse the spinal and the epidural space. There is normally a sufficient gap between the spines to enable a needle to reach the epidural or spinal space.