Pectinate muscles

The pectinate muscles of the right atrium in dogs are prominent ridged bundles of cardiac muscle located within the right atrial appendage (auricle) and extending to a lesser extent into the laterocranial portion of the right atrial wall.

They originate from a distinct muscular ridge called the crista terminalis, which runs longitudinally along the inner wall of the right atrium and marks the boundary between the smooth-walled sinus venarum and the roughened atrial appendage. From the crista terminalis, the pectinate muscles radiate in a comb-like (pectinate) pattern toward the free wall of the atrium and the auricle.

Structurally, these muscles are composed of myocardium continuous with the atrial wall. Functionally, they increase the contractile efficiency of the atrium without substantially increasing cardiac mass, helping to propel blood toward the right ventricle during atrial systole. They also contribute to maintaining atrial shape during filling and contraction.

In dogs, the pectinate muscles are generally well developed compared with the left atrium, reflecting the embryologic origin of the right atrium and its appendage.