The pectoralis major is a thick, flat muscle covering the upper front of the chest, beneath the upper inner portion of the breast gland. The lower outer portion of the breast is on top of another deeper muscle, the serratus anterior. The pectoralis major has a broad origin from the collarbone above, the sternum (breastbone) in the middle, and the ribs below. Pectoralis muscle fibers converge upward and laterally in front of the ribs to form the front border of your armpit, then insert on the upper inner arm bone, the humerus.
Breast implants are not natural, but are synthetic mechanical devices. They do not always appear or behave 100% naturally. Deeper implant placement helps camouflage implant-related imperfections. One popular technique for deeper placement is to lift the lower part of pectoralis major muscle, develop a pocket for the implant beneath this muscle, and place the implant within the “submuscular,” or “subpectoral” pocket. This actually results in a “dual plane” position, meaning the upper inner portion of the implant is beneath pectoralis muscle, while the lower outer portion, because it extends below the lower margin of pectoralis muscle, is beneath the breast gland.