Harold had a mild stroke due to a blockage in his Posterior communicating artery. He fully recovered because he got to the hospital within one hour. The neurologist stated the his quick recovery was also due to the fact that this artery was part of the "Circle of Willis". What is the Circle of Willis and why did the doctor say this? Include anatomical structure in your explanation.

Harold had a mild stroke due to a blockage in his Posterior communicating artery. He fully recovered because he got to the hospital within one hour. The neurologist stated the his quick recovery was also due to the fact that this artery was part of the "Circle of Willis". What is the Circle of Willis and why did the doctor say this? Include anatomical structure in your explanation.