Not a boundary of Triangle of Auscultation:
Serratus anterior is inseed on the medial border of scapula but lies anterior to scapula. Hence, it is not in the triangle of auscultation. Triangle of Auscultation is bounded by 2 muscles and scapula. Superiorly - Trapezius, Inferiorly - Latissimus dorsi and Laterally - medial wall of Scapula. Rib 7 and Rhomboideus major lie in the floor of the triangle. Since minimal muscle fibers lie over the triangle, auscultation by stethoscope is better over this triangle, especially, the sounds of swallowed fluids. Cardiac end of the stomach lies deep to this triangle.
Scapula
Trapezius
Latissimus dorsi
Serratus anterior
Back region