Which part of the CNS is described as a fragile structure extending from the end of the brain stem?

The spinal cord is indeed a fragile structure that extends from the end of the brain stem. It serves as a crucial pathway for transmitting signals between the brain and the rest of the body. The spinal cord runs down the vertebral column, protected by the bony vertebrae, but it remains vulnerable due to its delicate nature.

As part of the central nervous system (CNS), the spinal cord is instrumental in reflex actions and processing sensory information, functioning as a conduit for motor signals to the muscles. Its connection with the brain stem is particularly important for integrating various vital functions like breathing and heart rate.

In contrast, the cerebral hemispheres encompass the largest part of the brain responsible for higher cognitive functions. The medulla oblongata, although part of the brain stem, does not extend beyond it and is primarily involved in autonomic functions. The cerebellum is located at the back of the brain and is critical for coordination and balance, but is not a continuation of the brain stem. Thus, the spinal cord is the part of the CNS that best fits the description provided in the question.