Which of the following organs is NOT a principal organ of the nervous system?

The heart is not considered a principal organ of the nervous system. The nervous system primarily consists of the brain, spinal cord, and nerves, which work together to transmit signals throughout the body, process information, and coordinate responses to stimuli.

The brain serves as the central processing unit, coordinating thoughts, emotions, and bodily functions. The spinal cord acts as a vital communication pathway between the brain and the rest of the body, facilitating reflex actions and transmitting sensory and motor signals. Nerves, which are bundles of axons, extend from the spinal cord to various parts of the body, enabling communication between the central nervous system and peripheral bodily structures.

In contrast, the heart is part of the circulatory system and is primarily responsible for pumping blood throughout the body. While the nervous system does have a role in regulating heart function through autonomic control, the heart itself is not an organ that processes or transmits information in the same way that the components of the nervous system do. This distinction makes it clear why the heart is the correct answer in this context.