Which structure serves as an extension of the brain and carries messages between the brain and the rest of the body?

The spinal cord is a crucial structure that functions as an extension of the brain, facilitating communication between the brain and the rest of the body. It transmits nerve signals to and from the brain, playing a vital role in reflex actions and a variety of motor control and sensory processing functions. The spinal cord is protected by the bony vertebral column and is responsible for relaying messages that enable bodily responses, such as moving a hand away from a hot object.

The other structures mentioned do not serve this function. The superior vena cava is a major vein that carries deoxygenated blood from the upper body to the heart, rather than relaying nerve signals. The sternum, known as the breastbone, is a flat bone that serves as an attachment point for ribs and does not participate in nerve signal transmission. The stratum corneum is the outermost layer of the skin that provides barrier protection and does not communicate messages between the brain and the body. Thus, the spinal cord clearly stands out as the structure that connects the brain to the rest of the body through neural pathways.