Musculi pectinati

Overview
In the right atrium, behind the crest the internal surface of the atrium is smooth, while in front of it the muscular fibers of the wall are raised into parallel ridges resembling the teeth of a comb, and hence named the musculi pectinati (pectinate muscles).

In the left atrium, the musculi pectinati, fewer and smaller than in the right auricula, are confined to the inner surface of the auricula. This is due to the embryological origin of the auricles, which are the true atria. Some sources cite that the musculi pectinati are useful in increasing the power of contraction without increasing heart mass substantially.