The auricles (atrial appendages) are small, ear-shaped pouches that project from the upper portion of each atrium. Extending into these pouches are are parallel ridges of muscles that look like the teeth of a comb, which gives them the name pectinate muscles. In the median wall of right atrium is the fossa ovalis, a remnant of the fetal formen ovale which allowed blood flow to bypass the lungs.
An interventricular septum separates the right and left ventricles. Trabeculae carneae are ridges of muscle in the ventricles (not to be confused with the papillary muscles that are attached to the chordae tendineae of the atrioventricular valves). These increase the strength of contraction with minimumal additional muscle mass.
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