Which of the following is not an organ of the central nervous system :

Options

A. Brain
B. Spinal cord
C. Medulla oblongata
D. Vagus

Answer

To determine which of the following is not an organ of the central nervous system, we need to identify the components of the central nervous system (CNS). The CNS is primarily made up of the brain and spinal cord. Let's analyze the options provided:

1. **Brain**: The brain is a crucial part of the central nervous system. It is responsible for controlling thoughts, memory, speech, and voluntary movements. Therefore, this option is part of the CNS.

2. **Spinal Cord**: The spinal cord is another essential component of the central nervous system. It transmits signals between the brain and the rest of the body. Hence, this option is also part of the CNS.

3. **Medulla Oblongata**: The medulla oblongata is a part of the brain located at the base of the brainstem. It controls autonomic functions such as respiration and heart rate. Thus, this option is also included in the CNS.

4. **Vagus Nerve**: The vagus nerve is a cranial nerve that extends from the brain to various organs in the thorax and abdomen. It is responsible for transmitting sensory information and controlling certain involuntary functions. However, it is not classified as an organ of the central nervous system.

Based on this analysis, the correct answer is that the **Vagus Nerve** is not an organ of the central nervous system.

### Summary:
- **Correct Answer**: Vagus Nerve (Option 4)