Which of the following structures is NOT part of the arterial circle?

• A. Pituitary gland
• B. Thalamus
• C. Cerebral arteries
• D. Basilar artery

Answer: (B)

The thalamus is indeed not part of the arterial circle, also known as the circle of Willis. The circle of Willis is a complex vascular structure located at the base of the brain, formed by anastomoses between certain major arteries, specifically the internal carotid arteries and the basilar artery. It provides collateral blood flow to the brain, improving its vascular resilience.

The other structures mentioned in the question directly participate in the formation or structure of the arterial circle. The pituitary gland, while not a vessel itself, is located near the circle of Willis and receives blood supply from branches that arise from it. The cerebral arteries, which include the anterior, middle, and posterior cerebral arteries, are key components of the circle, supplying blood to the different lobes of the brain. The basilar artery is also crucial as it forms from the union of the vertebral arteries and contributes to the circle by connecting with the posterior cerebral arteries.

Thus, the thalamus, being a deep brain structure primarily involved in sensory and motor signal relay rather than blood supply, is not included in the arterial circle, making it the correct answer to the question.