Rouget cells surround the walls of

What are the cells that form the wall of a blood vessel called?

Which among the three layers of blood vessel wall—$Tunica \text{ intima}$, $Tunica \text{ media}$, and $Tunica \text{ externa}$—is comparatively thin in the veins?

What are blood vessels composed of?

Which of the following is different from others due to the absence of a muscular coat?

The following diagram shows a schematic outline of human blood circulation. Identify the blood vessels $P, Q$ and $R$.