Sternocostal head of pectoralis major muscle

From this extensive origin the fibers of the pectoralis muscle converge toward their insertion;

• those arising from the clavicle (clavicular head) pass obliquely downward and outwards (laterally), and are usually separated from the rest by a slight interval.
• those from the lower part of the sternum, and the cartilages of the lower true ribs, (sternocostal head) run upward and laterally, while the middle fibers pass horizontally.
• those from the sheath of the straight abdominal muscle (abdominal part) run more upward and laterrally.