Left auricle

The left auricle (auricula sinistra) in domestic animals is a conical, blind-ended muscular diverticulum that arises from the cranial aspect of the left atrium.

The left auricle is positioned on the left side of the heart base, partially overlapping the root of the pulmonary trunk. It lies cranial and slightly dorsal to the left ventricle and forms part of the (left) auricular surface when viewed laterally. It is narrower and more tubular than the right auricle.

It projects ventrally on the midauricular surface and lies caudal to the conus arteriosus and pulmonary trunk. Its apex covers the proximal end of the paraconal interventricular groove.

Externally, it is smooth and covered by epicardium (visceral pericardium), beneath which run branches of the left coronary artery and accompanying veins within the subepicardial connective tissue.

The internal surface of the left auricle is characterized by prominent pectinate muscles (musculi pectinati), which are parallel, comb-like muscular ridges extending from the auricular wall toward the atrial lumen. These ridges are more numerous and closely spaced within the auricle than in the smooth-walled body of the left atrium. The transition between the rough auricular portion and the smooth atrial wall is gradual.

Medially, the left auricle communicates directly with the cavity of the left atrium through a wide atrial orifice without a distinct anatomical boundary. Dorsally, it is adjacent to the left pulmonary veins entering the left atrium.

The left auricle represents the remnant of the primitive atrium and serves to increase atrial capacity and modulate left atrial hemodynamics.