Name the ridged bundles of muscle found projecting inside the right atrium.

Pectinate muscles

Explanation:

The right atrium is the upper chamber of the heart that receives deoxygenated blood from the body and pumps it to the right ventricle. The inner wall of this right atrium is made of muscular fibers. These muscular fibers are raised to form parallel ridges that looks like the teeth of a comb, which projects inside the right atrium. These bundles of muscle are called the pectinate muscles.