Final answer:
The fascia separates the breast from the pectoral muscles, which include the pectoralis major and minor, and are part of the pectoral girdle. This girdle, made up of the clavicles and scapulae, allows for the extensive mobility of the arm.
Step-by-step explanation:
The tissue that separates the breast from the underlying pectoral muscles is primarily made up of the fascia, which is a thin layer of connective tissue. The pectoralis major is a thick, fan-shaped muscle that covers much of the superior thorax, and the pectoralis minor is a muscle located beneath the pectoralis major that moves the scapula and assists in inhalation.
These muscles are part of the pectoral girdle, which also includes the clavicles and scapulae, and provides a stable base for the movement of the arm. The pectoral girdle muscles that help stabilize and position the girdle are located on the anterior and posterior thorax.
These include the subclavius, serratus anterior, trapezius, levator scapulae, rhomboid major, and rhomboid minor. The pectoral girdle, by allowing for extensive mobility, enables the humerus to move in multiple directions through its ball-and-socket joint with the glenoid fossa of the scapula.