The right answer is pectinated muscles.
The right atria extends between the two cellar veins. It is of rounded aspect, but for the simplification of its description, it is assimilated to one cube having 6 walls:
Anterior wall
A lower wall
A posterior wall
A top wall
An inner or septal wall
An outer or straight wall: relatively thin, has muscle thickenings: pectinated muscles. This is the preferred route of choice for cannulations during cardiac surgery under extracorporeal circulation.