What part of the central nervous system runs within the backbone of mammals?

The spinal cord is the correct answer because it is a vital part of the central nervous system that is housed within the vertebral column, commonly known as the backbone. It serves as a major conduit for information traveling between the brain and the rest of the body. The spinal cord is protected by the bony vertebrae, which form the backbone, and its primary functions include transmitting nerve signals and coordinating reflexes.

The brain, while also a crucial component of the central nervous system, is located in the skull and not within the backbone. A nerve bundle refers to a collection of nerves outside the central nervous system and does not specifically refer to an internal structure. The cerebrum is part of the brain, responsible for higher functions such as thought and action, and like the brain, it is not located in the backbone.