What is the structure of the spinal cord that resembles a "horse's tail"?

The structure of the spinal cord that is described as resembling a "horse's tail" is known as the cauda equina. This term specifically refers to a bundle of spinal nerves and nerve roots that extend beyond the end of the spinal cord. In humans, the spinal cord terminates at the level of the lumbar vertebrae, and below this point, the individual spinal nerves continue to travel down the vertebral canal, resembling the hairs of a horse's tail.

Understanding the cauda equina is important because it plays a crucial role in transmitting nerve signals between the lower limbs and the pelvic organs. The other structures mentioned do not fulfill this description. For instance, the medullary cone is the tapered end of the spinal cord itself, while the filum terminale is a fibrous extension of the pia mater that anchors the spinal cord to the coccyx. Denticulate ligaments are extensions of the pia mater that help to stabilize the spinal cord within the vertebral column but do not contribute to the horse's tail-like appearance.