It is important to note in the diagram above that the brain and spinal cord are coloured in orange and the nerves and ganglia in shades of blue. Before attempting to clean up this jigsaw piece I had erroneously assumed that the spinal cord goes all the way down to the bottom of the spine, but it doesn’t, it ends at the level of vertebrae L1-L2. After the spinal cord terminates, the central nervous system continues as a bundle of nerves called the cauda equina. The spinal cord is housed in a protective sac, called the thecal sac (often known as the dural sac), and this sac continues down to S2 in order to protect the nerves of the cauda equina.
The nerves of the cauda equina serve the lower half of the body, and branch out from underneath the spinal cord like a ‘horse tail’. The latin name for ‘horse tail’ is cauda equina, hence it’s name.
On the diagram above you will see a dark blue section underneath the spinal cord, which is the continuation of the thecal sac. This dark blue section of the thecal sac is the protective housing for the nerves which comprise the cauda equina, and contains the same fluid that bathes the spinal cord, the cerebro spinal fluid. The cauda equina is made up of paired nerve roots (sensory and motor), which are bundled together in one nerve root sheath. Sensory nerves gather information about the environment (eg touch, temperature) and send it to the brain, and motor nerves tell muscles to contract, thereby making you move. You will see that the nerves exit the thecal sac at different levels. At each level there is a pair of nerve roots contained in one sheath exiting left, and another pair of nerve roots contained in one sheath exiting right.