Pectinate mm.

The pectinate muscles (musculi pectinati) are parallel ridges on the inner surface of the walls of the right and left atria of the heart. They resemble the teeth of the comb, hence the name (which derives from Latin pecten, meaning "comb" or "rake").

Pectinate muscles are found on the internal surface of the anterior wall of the right atrium. Posteriorly, the internal surface of the right atrium is smooth.

In the left atrium, the pectinate muscles are fewer and smaller than in the right atrium. This is due to the embryological constraints. Some sources posit that the pectinate muscles are useful in increasing the power of atrial contraction without increasing heart mass substantially.

The pectinate muscles should not be confused with trabeculae carneae, which are found on the inner surface of the walls of the right and left ventricles.