The pectoralis major is a large, broad, fan-shaped muscle located on the front side of the thorax. It is the biggest muscle of the pectoral region. It travels across the front of the underarm, where it stays thick, and afterwards connects to its slim, square tendon of attachment, which allows it to enter underneath the deltoid towards its attachment. No muscular fibers of the pectoralis enter underneath the deltoid just as its tendon does.
Pectoralis major muscle is thin fan formed and emerges by two heads, viz Small clavicular head and Large sternocostal head.
Clavicular head – Emerges via the medial half of the anterior part of the clavicle.
Sternocostal head – Emerges via:
- Lateral half of the anterior surface of the sternum, around 6th costal cartilage.
- Medial parts of 2nd-6th costal cartilages.
- Aponeurosis of the external oblique muscle of the abdomen.
- Pectoralis major is placed by a U-shaped (bilaminar) tendon on to the lateral lip of the bicipital groove.
- The two laminae are inferiorly constant with one another. While posterior lamina is created by sternocostal fibers and the anterior lamina of the tendon is created by the clavicular fibers.
- The lower sternocostal and abdominal fibers in their route towards the attachment are twisted in such a way that, the smallest fibers are placed highest. This twisting of fibers creates the rounded axillary fold.
The pectoralis is divided into clavicular and sternocostal portions. The most lateral and inferior division of the sternocostal portion are called the abdominal portion.
Typically, the pectoralis major is flat, planar, and squared-off; however it can also be rounded and protuberant, depending upon its variable lineation and the growth of its muscular mass.
The pectoralis major creates the thick muscular anterior wall of the underarm.
The deep and hidden pectoralis minor is located posteriorly, increasing density towards the medial end of the wall of the underarm. A small part of the pectoralis minor might really involve the surface in between the pectoralis major and the uppermost noticeable digitation of the serratus anterior.
Nerve supply is by lateral (C5 to C7) and medial pectoral (C8 and TI) nerves. The only muscles of the upper limb that are supplied by all five spinal sections that create the brachial plexus, are the pectoralis major and pectoralis minor muscles.
Sometimes a vertical sheet of muscle fibers stretching via root of the neck to the upper portion of the abdomen enter superficial to the medial portion of pectoralis major. It is called rectus sternalis or sternalis muscle.
The clavicular head bends the arm, while sternocostal head adducts and medially turns the arm.
On raising a heavy rod, the clavicular head ends up being prominent when one efforts to lower the rod, the sternocostal head ends up being prominent.